Must be an American thing then, I'm European, for me "bro" is just short for brother and it is a term of endearment. It's not hard for english speaking humans all around the world to arrive at a natural abbreviation such as "Bro" without being exposed to isolated pockets of misuse. We certainly did.

I'll continue to use the word "bro" because in my timezone (and on a few on either side) it's not a term of offence.