[mcpackieh]

> I can easily imagine a culture where it's only OK to steal from friends.

It is easy to imagine contexts in which taking something from your friend is morally permissible, because in that context you know the other person won't mind that you've done so. But because they don't mind, it's no longer stealing. Taking becomes stealing when you should reasonably expect the other person to feel hurt by the taking. Hurtfully taking things from your friends is universally immoral in all cultures, but different cultures have different expectations and standards for what kind of taking causes hurt feelings.