First: "Rational" and "irrational" describe mental processes, not conclusions. A social rule can be useful or useless, beneficial or harmful, well- or ill-defended ....

("If deleting posts that propose violence would benefit Less Wrong, I want to believe that deleting posts that propose violence would benefit Less Wrong. If deleting posts that propose violence would not benefit Less Wrong, I want not to believe that deleting posts that propose violence would ...")

Second: Consider the difference between "we're not assholes" and "we don't want to look like assholes".

Or between "I will cooperate" and "I want you to think that I will cooperate." A defector can rationally conclude the latter, but not the former (since it is false of defectors).