Hi, 

Here is a reddit post about this issue..

Basically, (imo) it's not so nice to filter out usernames like "ThisIsABanana" or "CRISIS_ALERT" because of 4 random letters which just happen to also have another meaning.. and maybe there's a better way to do this... 

Especially, also considering this:

Mojang has always had a pretty relaxed stance on usernames. [...] Implementing a comprehensive username filter is quite difficult, and doing so would diminish the openness that has made the game so popular.

Thanks for consideration