Long Live the Confederation!
This kind of thing is (was?) permitted in lots of situations. It's pretty much a necessity. The classic example (to which, via Das Boot, the movie was of course referring in this situation) would be on board a submarine. What do you do with a mutineer when you have nowhere to lock him up? And even when you do have a brig, in some situations, not killing the mutineer may make things worse - costing further lives. Discipline, in wartime, is a matter of life and death.
It may happen, but it isn't *permitted* - the Geneva Convention (which also applies to the Confederation, per the Wing Commander IV novelization) says that a trial is one of the basic rights afforded anyone involved in a conflict.
In this particular case, you can't look at this situation and claim that Angel would have been punished for it. Common sense indicates, based on what we see in the scene, that Angel's within her rights. Regardless of whether a US Navy officer is allowed to summarily execute a mutineer in wartime, a Confederation officer very clearly is allowed to do so. Would there have been other consequences? We don't know - we can imagine that there would have been an investigation in order to confirm whether Maniac's death sentence was justified. However, I would also imagine the investigation would probably confirm that it was - after all, his mutiny resulted in the death of another pilot.
Moreover, I think you're ignoring the spirit and the purpose of the scene in order to make some sort of academic argument. It's pretty clear that the audience reaction to Angel threatening Maniac isn't supposed to be 'yup, the harsh realities of space warfare require this type of justice' (more to the point, Blair specifically replies: "this is wrong, Angel, and you know it").
Also... was that JW black label I saw Maniac pull out on hunter? That is the best scotch there is, I have a case of the stuff and I dont drink anything else. But I'm surprised that didnt leave anyone hammered. Thats ten year old scotch, strong stuff.
Yes, it is - the movie's credits give an 'Additional Thanks' to the Johnnie Walker company. I don't have a copy on me at work, but I *think* the novel actually specifies the brand ("My close personal friend... Mr. Johnnie Walker").