If we had allowed a "Free Mexican Army" to operate along the Mexican border, gave it humanitarian supplies (tents, food, medical supplies) and allowed it to freely cross the border, I suspect we'd have a different conversation. There's been a bit of this in Latin American over the last century, with the practice being either hideously immoral and an overt act of aggression or a wonderful act of enlightened political activism, depending upon which government and paramilitary group's under discussion. It's always billed as a kind of eternal moral principle, whichever stand is conveniently taken at the moment.
I also would probaby not have a problem if Nogales or Brownsville had a mortar or two hit them if we were actively sponsoring insurgents. After all, we'd be actively supporting an army engaged in hostilities against the country's government. That makes us allies of an enemy, and alliances have consequences. Just ask Europe in 1914.
Then again, we'd probably just invade Mexico and establish a government we liked rather than let an insurgency handle it. In this we'd receive thundering applause from all sections of the US population and the forces of world progressivism.
I guess the difference is that the US had imperialist if not colonialist aspirations in Latin America. Unlike the Turks, that indigenous group that left the Middle East in absolute peace for every second of the last 900 years and made no attempts to impose any sort of political, religious, or economic hegemony over anybody else in the area. It's a point in Turkey's favor that there's not a bit of nationalism in the current government's political make-up.