They should just remove the "M" rating entirely. It causes too much confusion, as it stands for "Mature" and people naturally assume that means graphic sex and violence can be included. What they really allow is Teen rated work and their TOS should reflect that. As for underage users, it's the parents' responsibility to moderate what their kids watch, listen to and read. I'm tired of these permissive morons expecting the world to bubble-wrap itself because they're too bloody lazy to actually parent their own offspring. Plopping your kid in front of a TV and going about your business is not childrearing, and the term "it takes a village" should not be taken so literally.
That being said, this can only be a good thing for AFF.net, and I hope people begin leaving more feedback to encourage writers to keep sharing their work here. One of the (only) great things about Fanfiction.net was that one is guaranteed to get some kind of helpful feedback from the audience, which helps authors determine what can and should be improved and what they're doing right. Sadly, that kind of feedback is rare here and on most other fanfiction sites.