Because if they don't want to be replaced, you're going to have a hell of a time replacing them. No matter what you put in the newsgroup charter, moderators effectively control the newsgroups they moderate, and no one's going to take the newsgroup away from them unless they do something egregious. And even then, it will be months of fighting over it. If you don't trust the moderators, don't vote for the newsgroup, no matter how good it sounds. You might be able to talk them around, but you're probably going to just have to swallow your rejections and put up with it.
Moderators reject posts.
That's the whole point of a moderated group. Don't think it's never going to be you too. If you have anything else to add to this post, or any suggestions about it, don't hesitate to mail me. Thanks to Tim Ottinger for the initial framework of this post, Gary Johnson for bugging me until I finally got it written and for working to make people aware of these problems, and many news.groups readers for suggestions and feedback.