Well, actually it's more complex than that.  For instance, the Romans would let certain newborns die of exposure.  Would it be rude to say something to them about it?

Anyway, generally speaking though, I get what you're saying.   In fact, nosy and bossy people are not helping cross-cultural relations.   For instance, it's not my place to criticize misogyny in the Arab World and really, what can I do really do about stuff I don't like, and who am I to stop stuff that others feel are not oppressing them anyway?