It's just a stereotype among all other war game/movie stereotypes. Like MF said, it's a lack of imagination; using established conventions as an easy, paint-by-numbers way of characterization. The worst example I ever saw was that awful show Over There about Iraq. You had the no-nonsense, tough as nails Sergeant, the intellectual white guy, and the former black gangbanger on the SAW.
What's ironic about this is, although the SAW gunner is not black, the 'Miles' character's nickname is "8-Ball", the go-to nickname for black characters in Vietnam movies.