To me it is not obvious.
All these boards, in different countries, are roughly "advertising" (as you put it) for these various companies. Why is it OK to write **** on an advertisement in Sweden but not in America, if it is completely unrelated to American moral standards? What drives them to censor their own board? What are they scared of?
Note that I am not claiming it to be written in law either way, I know they're 'voluntarily' censoring us (which you laughably refer to as their "liberty" to do so, unless you can find a place where self-censoring isn't allowed), just that it is what I believe Obsidian conforms to: they are scared of what the general American consumer thinks of them.
OMG! It's called private property rights, a novel concept I know.