Well I would point to a few points:

1. Yahoo doesn’t seem to take spam very serious.
2. I suspect a large number of spam attacks are denial of service attacks, I have noticed a lot of people getting emails concerning a desire to share child porn. 30,000 of these will result in 10,000 abuse claims to an already swamped customer care division. The filter system, with people being reviewed and marked safe, and the new China Flickr coming a week after the Filters is just the kind of target your radical hack activist can’t refuse.
3. If you site is marked as unsafe or restricted by Flickr admins not only can you show all the porn you want, but you won’t get spammed.

I assume with Yahoo’s China policy and the fact that all Flickr accounts are now reviewed by Flickr staff people will continue to try and exploit the system with spams which they know will result in abuse claims. Abuse claims flood the already taxed review system which causes Yahoo accounts to be deleted which then leads to more Yahoo accounts that mean more accounts need to be reviewed.

I am not a hacker, I would rather make than destroy, but if I was in to trying to spam down a site I would find it very hard to resist trying to set the filtering reporting system off in Flickr and bringing her down.

I am sure most of it is people spamming to try and make money, but I have seen forum comments about mass spams of such a disgusting nature that I can only assume someone is trying to use the system of user reporting of abuse and filtering to bring it down.