I think their ability to act would quickly disappear once a certain threshold of sheer numbers is met.
I would never count on that. Remember that northern Mexico, for all intents and purposes, is a war zone. Even if the government wouldn’t attack them, it would be a strong temptation for the cartels to do so.
Estimates set the death toll in the cartel wars are above 120,000 killed by 2013, not including 27,000 missing. By whatever logic the cartels would use, they would have no hesitation to kill even hundreds of gringos. Especially unarmed ones.