IMO, you can't just ban a genre of cosplayer from a con. Sure, it may not be "anime", but neither are all the video game cosplays there. Might as well ban all the Final Fantasy cosplayers as well.
Besides, i know a lot of people who dress somewhat steampunk on a day to day basis (not myself, but my boyfriend and all his friends... though i admit i have an outfit or two to fit in with them). Anime Expo is just a day for them to chuck on the pieces that would look weird on a normal outfit (like the golden arm brace and crazy monocles). But really, even if its not japan-ish, it's still just what they want to wear. And thats cool with me, as long as they don't go saying its from Naruto or something. Cuz its not.