The ability to change your username if you have no posts is a feature baked into Q2A (the software this site uses). You can only turn off username changes for people with posts.

Obviously it's within Pokemaster's power to change this, but he hasn't done work on the software for years.
I imagine the feature is baked in because no one expected it'd be a problem for people with no posts to change their username. But it's a bit different on our site with the chat room and triple-section setup.