The 1983, Mentzer version of D&D (a series of D&D rulebooks sometimes referred to BECMI) provided another take on the awarding of experience. It is interesting to see the evolution of experience awards. Although Mentzer D&D still focused on experience for treasure and monsters defeated, the explication was becoming ever more specific.
"Did you notice that you get a lot of experience for treasure, and not much for killing monsters? It's better to avoid killing, if you can, by tricking monsters or using magic to calm them down. You can sometimes avoid the risks of combat." (Players Manual, page 12)
As has been explained better elsewhere, the reward systems of early versions of D&D were all about searching for and looting ancient treasure hoards (that were often also guarded by fell beasts). If the looting of the treasure could be accomplished without having to face the "risks of combat", all that much the better.
And speaking of treasure, in Mentzer D&D there was a 1 in 6 chance that treasure would be in an otherwise empty room, a 2 in 6 chance of treasure in a trapped room, and a 3 in 6 chance of treasure in a room occupied by monsters.