Fun with relativism

I don't understand, really. I don't hold that relativism is necessarily the "correct" way of doing things, but I admit that I disagree with sending out our "humanitarian" missionaries to other countries with other cultures, and then complaining when they do something illegal is the wisest of ideas.

Consider: You travel to a distant planet. You get there and are thrown into prison because skeletons are banned and considered weapons. You knew this law existed on that planet. Who is wrong?

They must explain this stuff in college or something. To me the idea of "live and let live" is relativism. Is there something objective, something logical, that gives a sense that a is always wrong and b is always right? If not, then on what does anyone base any of their ideas on anything morally?

Or was the Holocaust and American Slavery justified? To Nazi Germany and the Plantation slave owners it was...