Who the heck is valve to say what usernames I can and cannot have on my system? I don't even play TF2. What happens if the cheaters decide the change the 'trig

Who the heck is valve to say what usernames I can and cannot have on my system? I don't even play TF2. What happens if the cheaters decide the change the 'triggering' username to something far more common? Guest, admin, local, user, etc. Now valve is allowed to ban everyone who has that username on their system?