The traditional murder motives apply: revenge, and eliminating rivals. Revenge seems like it would be the most likely motive.

There have been cases in which prosecutions were based solely on the use of a credit card number which the owner claims must have been stolen; those cases most likely involve card numbers which really were stolen, but from a random victim, not to frame someone in particular. However, those cases are publicly visible evidence that framing someone is easy, and at least some malefactors must have noticed.

People convicted would tend to maintain their innocence regardless of whether they were innocent or guity, so that can't be used to determine how many people were really innocent. Computer forensics aren't very useful, because a sophisticated attacker can modify all the evidence. One way to find out would be to pose as a black market buyer wishing to frame a (fictional) person, and ask anyone who offers to perform the service whether they have experience doing that sort of thing. However, that would provide only qualitative data, nothing quantitative.

To pump up arrest or conviction stats.

I also remember reading, several years ago, that most web sites offering child porn were actually run by the FBI. No way of knowing if it's true.