No argument there. My post said "Typically how this is supposed to work". It doesn't mean TVTropes works like that.
Their current terms of use also have no bearing on their legal right to change the licensing from CC-BY-SA to CC-BY-NC-SA. That depends upon what terms of use they had in place before the conversion.
Their current terms are: if you contribute, we own (the copyright to) all your contributions. That's not only unfair to users, it may even be illegal because in a contract you have to provide some compensation in exchange for the release of copyright.
Their current terms of use also have no bearing on their legal right to change the licensing from CC-BY-SA to CC-BY-NC-SA. That depends upon what terms of use they had in place before the conversion.
Their current terms are: if you contribute, we own (the copyright to) all your contributions. That's not only unfair to users, it may even be illegal because in a contract you have to provide some compensation in exchange for the release of copyright.