In one of the criticisms¹ posted at ‘Answering Islam’, the author has pointed out a contradiction in 4:15-16 and 24:2. The former verses read thus: 

And for the harlots among your women, call four witnesses, from among your people, against them. Thus, if they testify, then confine them to their houses until death claims them or God ordains for them another way. And for the two, who are guilty of lewdness from among you, punish them both [with a beating]. Then if they repent and [promise to] correct their behavior, then let them be. Indeed God accepts repentance most; Infinitely Merciful. (4:15-16)

    The latter reference reads as: 

For the male and the female guilty of fornication, flog them both a hundred stripes. Let not mercy for them overtake you in the implementation of God’s law, if you truly believe in God and the Final Day. And let the implementation of the punishment be witnessed by a group of believers. (24:2)

    The propounder of the referred criticism, Jochen Katz writes:

... There is the contradiction whether for a female adulteress the punishment is one hundred stripes (24:2) or confinement in the house until death (4:15). If 4:16 does not speak only about homosexual acts but also about adultery of man and woman, then another contradictory element is added: If they repent they can get off the hook without punishment? Who will not repent with the prospect of a hundred stripes waiting for them? Apart from the question whether the punishment should be as in 24:2 or 4:15, how come the man and woman be treated equal in 24:2, but seemingly different in 4:15?

    A thorough understanding of the two verses should sufficiently answer the questions raised by Katz. A brief explanation of the verses follows:
    4:15 relates to the crime of prostitution. This is precisely the reason why only women have been mentioned. The punishment for the crime stated is house arrest till death. Quite obviously, this form of punishment prevents the woman in question from spreading mischief and lewdness in the society. The last words of the verse – ‘until God ordains for them another way’ -- clearly implies that the stipulated punishment (of house arrest) is for a period of time after which when the society is prepared and organized for it, the permanent law shall be given. 4:16 relates to a male and a female involved in fornication. Immediately succeeding the words ‘until God ordains for them another way’, it was clear that the stipulated punishment was also effective for a temporary phase till the time that the society was prepared and organized for the permanent punishment to be given. The punishment stipulated in the verse is of an undefined beating, which is in keeping with the fact that during the initial phase, the punishment for the crime was not strictly prescribed. 
    The above explanation should also spell out why the punishment in the first case is more severe than that in the second case. The two crimes, as should be clear, belong to two different categories. The consequence of the latter form of crime is restricted to the two individuals involved in it, while the former, if allowed to flourish, results in the moral degeneration of the society. 
24:2 (and the verses that follow) prescribe the final and the permanent punishment for the crime of fornication. As far as the final and permanent punishment for prostitution is concerned, it is not given in 24:2. However, besides these verses, 5:33-4 prescribed punishments for crimes that were of the nature of spreading disorder and anarchy in the land or of religious persecution or in any other way affecting the well being of the society at a macro level. Explaining the penal law of Islam, I had written in my response to a question:

It must be understood at the outset that Islam, in its injunctions regarding the penal law, has classified crimes into two major categories:
 

• Normal crimes; and 

• Extra-ordinary crimes. 

Examples of the first category may include crimes against someone’s property, i.e. theft, crimes against someone’s life, ie, murder or physically harming a person and crimes against chastity and respect, ie, fornication and defamation. Besides this classification, Islam has given another class of crimes that may appear to be of the same nature as given above but are committed with an absolutely different psyche. For example, a person may quietly enter someone’s premise and steal something from there, and in another case, a few people may form a gang of robbers and rob market places, houses and automobiles on the highway. Now, if you would consider closely, although a crime has been committed against property in both the cases, but the psyche of the criminals is quite different in the two cases. The first case entails a psyche of a simple criminal while the second case entails a psyche of a person who wants to create unrest and disorder in the society as a whole. Like wise, in case of murder, a person may kill another person on provocation, in hot blood or only to ‘get even’ for an insult, or to settle the score of an old enmity. While on the other hand, a person may install a bomb in a public place and thereby kill anyone -- man, woman or child -- that comes within the range of the blast. Now, again, even though the two acts have resulted in the same thing -- that is loss of life -- but still the psyche in the two acts is absolutely different. The same is the case with crimes against chastity and respect. A man and a woman may indulge in the act of fornication or adultery, while in another case, a man may rape a woman or a woman may open a brothel. 

Crimes of the first category are basically against the person, property, respect and morality of individuals, while crimes of the second category are basically against the society, as a whole. The crimes relating to the second category are called fasad fi’l-ard in the Qur’an.²

    Regarding the punishment of fasad fi’l-ard, the Qur’an says: 

The punishment of those who declare war against God and His messenger and try to spread unrest in the land is nothing but that they be sentenced to a painful death or to crucifixion or that their one hand and one foot of the opposite side be amputated or that they be sent in exile. This punishment is a disgrace for them in the life of this world, and in the Hereafter, a great punishment awaits them. Except for those who repent [and correct their behavior] before you catch them. Be mindful that God is Forgiving, Merciful [for the repentant]. (5:33-4)

    Explaining the penal law of Islam, I had written in my referred to response:

Islam, in its penal law, has proposed two different kinds of punishments for the crimes of the two categories... the punishment for fasad fi’l-ard is mentioned in 5:34. In this verse, the Qur’an says that such individuals should be slain in a painful manner (taqtil) or should be crucified (taslib) or one of their hands and one of their feet should be amputated or they should be sent in exile (they should be removed from their society and thereby from the influence of bad company). It is obvious from this verse that the court has been given the authority to decide and implement from amongst these four punishments according to the gravity of the crime and according to the psyche of the criminal.²

    In view of the above explanation, it is clear that the final and permanent punishment of prostitution (for which a temporary punishment was stipulated in 4:15), rape etc. was derived from the given verse. Stoning to death was, in fact, one of the methods adopted by the Prophet (sws) for implementing ‘taqtil’ (slaying in a painful manner). Thus, the punishments mentioned in 4:15-16 were, in fact, given for a temporary period, during which the newly formed society was organized and groomed. During this time, lighter forms of punishments were administered to the criminals³. However, later on, as the moral standard of the society elevated and as it became more organized, stricter punishments were prescribed for these crimes. The latter punishments replaced and abrogated the previous order.