A good DM (even in an Evil campaign) shouldn't include such situations without providing the Right Answer to the dilemma, *in regards to the campaign world*.
An alternative approach, which I prefer, is to allow the players to provide a solution.
For example, in my current game, one of the players suggested that the PCs require the hobgoblins whom they had taken prisoner to swear oaths foreswearing warfare, and also accepting exile from the lands that they had been raiding. The other players went along with this, and in game the oaths were duly extracted.
If the PCs are willing to Do The Right Thing, the DM shouldn't be looking for ways to screw them up later.
I agree with this. I've not had any reason, as GM, to have those hobgoblin NPCs break their oaths.
I see this as similar to the standard GM response to PC victory in combat - most of the time, the opponents don't come back to life to harass the PCs.
if there are disagreements among the players, because the DM never determined what is customary in the campaign world, then hours will be lost in pointless argument
Hours of argument are rare in my game, but part of what playing the game invovles is the players debating what should be done, and why.
Sometimes they don't agree. This has resulted in unilateral decisions to execute prisoners, or in prisoners being handed over to the authorities when other PCs would have liked them dead.
Otherwise he's just setting up the campaign to watch the world burn.
At least with the right group, a campaign can survive the occasional disagreement between the players over what should be done.
In my view, the GM asserting authority over this doesn't actually solve the problem, because if the players don't all agree with the GM's moral judgement, the same issues will arise.
Better, if a particular moral question is likely to provoke game-breaking disagreements, for the GM and players all to work together to avoid introducing such situations into the game.