Here is my take... at least in Mexico the violence is directed toward a specific target. In the US, that target could be random from the guy who's wife cheated on him to the guy who is angry at the world. Often times the police will look the other way in Mexico when one of the bad guys falls out of line, why... because someone within the criminal organization will take care of the rouge employee.
As the pressure has increased on the acquisition and transfer of drugs to and across the border, many of the cells have experienced a marked reduction in income and have resorted to other means of generating income to pay the sometimes extensive staff of little people, halcons, etc. If a cell loses a load, they have to pay the boss the value of the load. Those alternative means of generating income include, collecting protection, kidnapping, abducting young women for prostitution and a few other activities. The victims are mostly local residents but often not affiliated directly with any cartel cell. Visitors are pretty much spared, unless they look Mexican and or have business in an area. Pretty broad specific target.
Also the idea that it is a good thing when one cell shoots up another cell or a rogue member, leads to lawlessness and enforcement becoming cop, jury, judge and executioner. Any civilized society investigates a murder, irrespective of who did the shooting. Murder is murder. Overlooking it creates a permissive atmosphere.