they ban certain guns because they are way overpowered so they take it into their own hands to balance the game since most of these companies are too stupid to balance the games themselves.. its been like this years before MLG was around. If you ever heard of TWL or Caleague they used to have server settings designated for each game that would balance the game for competitive play.
Same with tournaments
They do it with maps, settings, kits, weapons, attachments anything to balance the game
it doesn't balance the game... the game is the game..MLG was gamebattles.com back in the day i believe