Expectations of politeness generally involve letting people have their happy delusions if they aren't harming anyone. For instance for someone whose son went MIA in Vietnam and holds hopes that he will return one day telling them "He probably either died and his corpse was forgotten or in the off chance he is alive went AWOL with some peasant girl after getting fed up with the war and lives in fear that he will be found out.". Even if statistically it is 100% true that would be staggeringly rude to say to them.