It is a matter of scale. They are asking you to trust a couple founders of the company whereas you are asking them to trust thousands (potentially millions if they are successful) of users. Better to have Facebook filter out most of the bad actors up front and not pollute their site with spammers, trolls and other shenanigans until Facebook-only login is retarding their growth.
It works the other way around. Facebook is retarding their growth right now, when they can't afford to have it retarded. On the other hand, when they are big, they can afford to make users jump through hoops (and identify themselves).
That's really not true - Facebook, when used properly, can be an astoundingly good driver of growth. If you're not the type who would use Facebook, you're probably not the type of person who will drive their growth anyway, so why should they take away time from optimizing for those users who will?
