i dont know who started this whole thing, but this wasn't the first time it's been done; i think it's just an excuse for cosplayers to try and find other local cosplayers and meet up outside of a con setting. Starbucks was probably just chosen because they're everywhere and make a decent meeting place.
Not everyone who cosplays in public is a retarded weeaboo. There's an unfortunate majority that sets a bad example for our hobby out there, that doesn't mean the rest of us can't set a GOOD example instead.
I mean really, at some point, someone who's not involved with a convention will always see you in your cosplay, you have to walk from your hotel to the con and whatnot right? there's no need to get your panties all in a twist because some people are gutsy enough to do this and don't mind the stares they get. Some people only get to go to one convention a year, it doesn't hurt to have another situation where you can cosplay.