i'm speculating, however i think the idea is to ID troublesome users and prevent them from opening up a 2nd account. however, you don't need to keep an account since you can IP ban in IPB without keeping a user account. and not that it would truly be effective anyway since its simple to access the site with a different IP like Terry mentioned anyway.
65000 accounts looks a helluva lot better than 12000 to a potential advertiser though.