I just self-banned at IIDB two nights ago, so I guess I understand it.
I sometimes find discussions there really interesting, so I go there fairly often. However, as an opinionated person I almost invariably end up getting in an argument with someone, and a fairly high percentage of the threads I post in end up getting locked. So, being someone who hates moderation and tries hard to obey the rules, I get annoyed.
Hence the easiest way for me to still derive whatever benefit I can get from going there at all while ensuring that I won't get into yet another boring argument that results in yet another annoying act of moderation, was to request to be banned so I can't post even if I want to.
So yeah, definitely self-parental control for me.