Freedom of Speech!!!!!
Doesnt apply. No really. If were talking about American law, the First Amendment only applies to the government making laws that prohibit speech and, even then, freedom of speech isnt absolute. For example, you cant incite a riot, plan to kill the president or yell fire in a crowded theater.
Also, theres nothing in the First Amendment that protects you from the consequences of your speech. People are fired all the time for making derogatory comments about their employer. My workplace, for example, has a social media policy that says that I can be terminated for badmouthing the company. They dont ask for passwords and access as some companies have been known to do, but it wouldnt be all that difficult to find me on the internet, knowing my real name, phone number and address.
http://forums.na.leagueoflegends.com/board/showthread.php?t=4086541&page=1#post43465469
Muting and chat filters aren't an excuse to talk the way you like either; players shouldn't have to plug their ears to pretend you are behaving, you just need to be sportsmanlike on your own. When they are forced to mute you, you've probably already created a negative experience for them, and censoring doesn't really apply to anyone but children that don't know what is being censored; when anyone older than that sees the word censored it has little effect because they know what goes there, or even worse because they're being made to think of the possibilities.