Chrissy, if you link to a Professor-rated fic, the screening stays in place to demand the login. It won't give anyone a way around that I'm-seventeen checkbox.
Zacko, this is the sort of question that belongs in a Room of Requirement or General Fanfic Discussion thread, not an ask-the-mods thread. However, one reason would be that a writer who has perfectly good grammar, punctuation, and spelling skills may still want a canon-checker, a plot-checker, or a long-sentences checker. Another set of eyes is always a good thing, and just because something made sense to the writer doesn't mean it'll work for anyone else.
Lily, that's just plain weird. All I can suggest is that the edit page should be used as seldom as possible (it messes up the formatting anyway) and that the category should be double-checked before you leave the first part of the submission routine -- it can get dislodged with a scroll down, for instance. I'll try to keep an eye out for your things in Marauder-Era and make sure they stay there, though.