I've only ever heard of new members being put on moderation- and after 6 posts, I think, or maybe it's 5, they are taken off it. this is to protect the forum from spammers.

as for other reasons, well, I think the best thing to do is probably write to an admin.

if a forum user is on moderation (and they are not new), my guess is that the admin has a really good reason for it- this site is big, and I'm also pretty sure that admins don't take those type of decisions lightly as it means extra work for them.

they will come off moderation eventually, it just takes time