Yeah, the reason is entirely to have a place to put content that makes advertisers, webhosts, and other wannabe moral guardian types nervous that isn't immediately available. If it takes a few weeks of diligent posting to unlock access, it's a lot less likely that they will notice and complain.
A lot of the loli/shota content in the main Games forum is tamer - the "petite 18" or "age unspecified" setup.