World of Warcraft has an ESRB Rating of “T”, meaning Teen - Ages 13 and up. On top of that, Blizzard has it’s own rules and terms in the ToS and EULA. One of those being that no cussing of any kind, written, abbreviated, masked, or in any form.
So, there are minors that play this game. The Mature Language Filter is in place as an extra layer of protection for those that want to protect their children or themselves from seeing what is typed by those that don’t seem to be able to follow the rules.