I think another reason could be the wide latitude you have in the military to punish someone without any official paperwork. If someone did something wrong, you could punish them with physical exercise, cleaning duties, taking away weekend liberty, etc., all without any paperwork. When you don't have to worry that you're going to ruin someone's career every time you punish them, it's a lot easier to keep them in line.
I was thinking the opposite: the US military has "up or out", where one has to advance to stay in, tending to weed out bad apples in the officer corps. As far as I am aware, police have nothing similar (and furthermore, don't rotate). So my assessment is that it's much easier to get along to go along in US police forces than in their military.
The big problem with up or out is that officers eventually become politicians (literally connected to politicians) with the pseudo-immunitiy that politicians seem to enjoy. Prosecutors just choose not to prosecute, or they just retire in lieu of what should be jail time.
