So, I can’t use my legal name as a username because some random town with a few thousand people is named the same?
In it's current state, I'd look at the API to check for reserved / premium names (or something that's profane).
If it makes sense contextually: imagine if you were building the next Twitter. I'm guessing you'd want to have a way to charge for premium names and in-turn need a way to detect what's premium. For the most part, first and last names are pretty premium and people pay (they do!) for such usernames.