The game is M rated, but the forums are not.
Read the forum's CoC if you don't understand why.
It's an M-rated game, meaning everyone posting on here should either be an adult or have the permission of an adult who should be filtering for his/her own child. Personally, I believe it is just polite to talk a certain way in public but beyond that trying to censor others in any public forum is ridiculous and even counter-productive in certain cases.
And, yes, even though Blizzard can claim this is their forum and their rules, it is still a public forum. If it is not a public forum, and actually a forum by invitation, then I'll go back to the original point which is this is a game rated M for Mature, so why should impressionable children be allowed on such forums (if they're considered Blizzard's private property) and the game has content only suitable for adults?
As far as what adults say, so long as no serious threats are being posted and nothing else illegal is allowed, where's the issue? Why should we be telling adults how to speak? Most nations involved in Blizzard's production of Diablo have freedom of speech do they not? Watch out folks, censorship is a slippery slope. Blizzard hasn't been *too* bad about doing it, please don't encourage them to get worse. That means defend anyone's right to rant or use foul language but avoid using foul language if you can.