Once in an AD&D 2nd edition game, after a not very successful encounter with wererats, our beaten-down 2nd/3rd level party — one character unconscious, both clerics exhausted — was traveling a road to get to the nearest town before dark, as it was said there were undead at night. A lone hungry bear appeared out of the forest, his intent to eat the party obvious. The mage and the elf-ranger used magic missiles and a bow, respectively. Surprisingly, they killed the bear before it got close.
The DM decided that the kill was too easy, as the bear was sick and had too few hit points, so the party got no (zero) experience points. The party argued that if the bear had reached them they would have had little chance, as they were beaten and exhausted, not to mention that the bear was possibly rabid, so the correct action of those two characters saved the party.
Is the DM right? Are there rules in AD&D 2nd edition that specify when an encounter is too easy and worth zero XP?
Background information: (1) The DM's argument of 'too easy' was backed by the fact that the party lost no hit points. (2) The DM was clearly bored having to run a game with novice players (for half of the players, it was their first game). He was annoyed with the need to repeat the rules to the players and to explain the game mechanics most of the time. But he was ambitions to run the game himself, even if he had to play with novices. (From comments under @PrometheanVigil's answer).