WWII, Vietnam, etc. all happened "out there somewhere", and aren't really considered by those who didn't actively participate. Veterans of such conflicts are acknowledged as being deeply affected, but the rest of the populace effectively shrugs as if to say "sorry, I don't get it", and go about their lives.
Terrorist attacks and Mexican cartel atrocities affect less than 0.00001 % of the population. (Assuming perhaps 3,200 people per year, which is quite a high estimate.) To the rest, it's just a special effect display or soap opera drama on TV. Street violence in poverty-stricken communities is the closest thing people have to experiencing violence in the US, but that only affects a relatively small portion of the country, and those with little influence.
tl;dr: I think violence is acceptable in US media because most Americans have no true comprehension of violence. It's all a video game or a movie to them.