Oh, the freedom of speech! Oh, our rights are being violated!!!
You have the freedom of writing everywhere you want, that doesn't mean you have the right to get paid!
That was my gut reaction when I read that part of OP's post, but I don't think he's saying anybody's rights are violated--that's my take on it at least.
As far as a campaign kicking you out for posting in certain sections: I've never seen that as a campaign rule though I'll admit that I haven't read through all of the rules of every campaign. But even if it was one of the rules, it's still completely fair. If the campaign owner doesn't want posts being made in certain sections (like maybe in Gambling), it's their right to either not pay you if you post in a forbidden section (which is usually the case) or to even kick you out of the campaign if you violate that rule.
And why?
Their money, their rules.
That's why. Be grateful these campaigns exist in the first place, because they may not be around forever.