A ton of users have invalid usernames, and this is because they were grandfathered into the system from before the rules were made. This is specifically for the rules that make sure your username won't break any site functionality for you. (So, things that affect other players - like having a bad word in your username - will not be treated the same way by staff, we would enforce the rules on those.)

For the invalid usernames due to underscores and the like, is correct. We found it fairest to offer you the choice: you can file a ticket to get a free username change done by staff at any time, or you can keep the invalid username with the understanding that some parts of the website may break for you as a result.