That's a good question! I have no idea.
Seriously, it just has to do with liability. We do not want any problems from hysterical parents or being accused of helping minors. If a kid was smart enough to lie about his age (or at least not promote it) and act like they were of "age" I don't think it'd be as much as an issue. We can say- "Well he SAID he was 22!" for example.
But in a case where someone comes on the forum and says they are hiding homebrew under the sink to hide it from their parents, and admits to being a teenager in the US, we have to act and can't turn a blind eye.
In cases of adults who may or may not be skirting the law on several issues, as long as they don't brag or post about it, it's not my problem.
I have to have a certain friend who gave me a certain liquid at a certain NHC. And I liked it, as that certain person knows! (That WAS you, wasn't it, chef?)
I'm pretty laid back and easy going, and wish the laws in the US would be changed. I'm fine with recreational drug use for adults, for example. But this isn't about my personal feelings, it's about not allowing discussions of illegal activities on our forum. Anyone dumb enough to say "Hey, I'm doing some illegal stuff" won't be long on our forum.