I won't speak to your own particular situation; there's not enough information to make any kind of judgement on whether the moderators are right or not. But as a general rule, I can think of at least one situation where a permanent ban is indeed the right thing to do. In the case of harassment, allowing a harasser back onto the site will often make the victim feel unsafe again. The victim doesn't know for sure whether the perpetrator has changed or has simply learned to say the right things to con the moderators into thinking that they have done so.
There's also the simple issue that if the moderators don't hold by the rules for one person, you'll get others complaining that, "if X was let back in, why shouldn't Y be?" regardless of how similar the situations may or may not be. It opens up a can of worms that the moderators may not want to deal with.
I do agree with Chris F that any site with moderation should keep a record of moderation decisions.
