For the same reasons that Aeria bans people for using 'inappropriate' language in a game. "It could offend people." (their #1 reason)
If you really want to type some profanity, just put an apostrophe after each word that would be blocked. Doing this makes it so the word isn't censored.
Doesn't work anymore. I used to do that on ijji.
Really I only swear when trolling anyways. Other than that I see no need to.
I'm a silent player so although I rage hard IRL, i never type "OMFG U STUPIDF**** NOOB OMFG LAG"