From what I am hearing, this purge is going to happen because the Linden company is planning on putting SL servers in overseas countries and they don't want to get in trouble with the foreign authourities.

For instance, one of the groups rumoured to be getting the chop are the ageplayers - users that deliberatly make their AV look child-like. Of course, some ageplayers will also use these AVs for sex. I'm not sure if the whole 'adult dressing as a child for sex' thing is illegal in the US, but it is in Australia - and the Lindens would be very silly indeed if they didn't -put a server down here, so they need to start complying to local laws too.