I don't think that the referees condone fouls or violent play. There is only so much they can do when players are playing out of control. If they blew their whistle and issued a card everytime the players, parents, or coaches wanted one, the game would never flow. Certainly, a player coming in like a linebacker or taking out a player from behind is an easy call. The problem is in sloppy, reckless play where bodies are flying and collisions are every 5-10 seconds. With poor buildup and possession, there are bodies going everywhere.
As far as the Mexican Leagues, I am also guilty of over generalizing. My son who is Caucasion has played in the Mexican Leagues since he was in 8th grade. We came from a town with only one select team and when that didn't work out that was his only choice besides rec. We were nervous at first. But once we got to know the culture, we became more at ease and now I worry very little about injuries in those games. Respect is important. As long as you are not playing to hurt anyone, then the players treat you with respect. For these four years, I've never seen a serious injury on the field and I have never seen anyone hurt at all from a fight. My son has only received a cheap shot once and players and fans from both teams scolded the player. In youth play, I see injuries every weekend. It's the way we play the game.