They didn't; e.g. playing the lottery as an adult is both legal and insanely idiotic. (Don't misunderstand me; I do like it that the insanely idiotic are able to self-select themselves away from their money with such ease.)

But on the other hand, "legally can and causes more utility gain per unit time than any other course of action" ... does indeed imply "should".

(And unless you're really silly, "utility gain" simplifies to "monetary gain".)

Of course, I'm not trying to imply that sabotage-of-beliefs is that powerful an action; indeed, my ultimate conclusion is that neither tire-slashing nor sabotage-of-beliefs present a compelling benefit-per-cost.

Yeah, that's why I always turn down my friends when they ask if I want to go see a movie. Monetarily, it's nothing but a loss, and I would have to be "really silly" to value an entertaining experience.