Because I care about having true opinions, I completely avoid film makers that in the process of producing and sustaining strong emotions in their audience rely heavily on beliefs about the real world without caring much whether those beliefs are true or false. Oliver Stone is one of those film makers. (I never saw any of his films. It was possible to learn enough about them to know I wanted to avoid them by watching a few trailers and reading a review or two.)
I prefer for my filmed entertainment to be completely fictional rather than to try to "teach" me about reality. When I want to refine my understanding of reality, I usually read nonfiction. It is too easy for filmed entertainment to "hack my brain".
Anyway, US bombers were forbidden to drop bombs within 15 miles or so of Haiphong harbor, probably because Russian ships were known to visit there regularly. Hanoi was also off limits for the vast majority of bombing missions. The directives forbidding these things came right from the top, i.e., from LBJ. In fact, LBJ would involve himself in the planning of individual missions, giving instructions such as approach this target from this direction because if you go in this direction, you fly over a school.
I'm not saying US soldiers and US pilots never committed atrocities in Vietnam, but the leaders of the US war effort were certainly never unrestricted in their willingness to kill North Vietnamese (although they probably would've chosen to kill every North Vietnamese communist combatant if there were a way to do that without killing any noncombatants). Consequently, it would've been rational for the US to continue the war effort in Vietnam even after it became obvious the war was unwinnable.
George Friedman says that the reason the US spent so much blood and treasure in Vietnam was . . . Charles de Gaulle. When de Gaulle was the leader of France, he was telling the other Western European nations that the US could not be relied on to come to the aid of Western Europe if the Soviets attacked. So, the US stayed in Vietnam to show Europe that a US military guarantee means something. Note that in order to achieve that goal, it was not necessary for the US to win the Vietnam war, only for it to try to hard enough and long enough. In other words, it was rational for the US to continue the war effort even after everyone realized that the US was unable to win the war (because the real goal was not to save South Vietnam from communism, it was to save Western Europe from communism).
The leaders of the US war effort certainly did not explain this reasoning in an effective way however to the American people -- maybe because there was no way to do so without severely demoralizing US soldiers, sailors and airmen.