One of the more common defenses I've seen for rude/discriminating behavior is justifying it with multiculturalism.
I know that sounds odd on its face, so let me explain...
Users seem to be justifying behaving badly by claiming that it's the norm in their neck of the woods. Sometimes that's even accurate, but I suspect, more often than not, it's just a lame excuse.
Admittedly, some places seem to be ok with some kinds of discrimination, it's a sad fact of the world. Some places have made a little more progress, and while that's great, they're still fighting to make it a little better... Stack Exchange seems to have become one of the latter places.
I'd prefer that Stack Exchange communities put their own culture first. As in:
We don't care where you come from, you still have to treat people like people here.
That seems to be a corner stone of the Be Nice policy, at least as far as my reading of it. Although this can apparently be interpreted more than one way.
To flatly come out and say:
It's not ok to treat X that way.
Can be considered rude in some cultures, if you're going by the standard set by a given culture. How they feel about how X should be treated is culturally ingrained, often backed by eons of tradition, religion, and so on. This obviously creates a big stumbling block...
What I would like to suggest is plainly saying that Stack Exchange is a community with its own culture, and the norms and rules of our culture come first.
We don't want to offend users from any part of the world, from any religion, or culture, but when it comes to some things... Well... That's just how we do it here. This is what our community feels is right.