In MMORPGS, the game and setting are usually asymmetrical by design - there's no assumption that the human knight should have an equal amount of hit points as the ancient dragon, and it would actually violate the logic of the setting if that were the case.

The games where people do complain about AI cheating tend to put the enemies in a more symmetrical role - e.g. in something like Civilization or Starcraft, the game designers work to actively maintain an illusion that the AI players are basically just like human players and operating under the same rules.

If you break that illusion too blatantly, players will be reasonably annoyed, because they feel like the game is telling them one thing when the truth is actually different.

This may even have in-game ramifications: e.g. if I'm playing against a human opponent in a multiplayer match, I might want to keep my units hidden from him so that he doesn't know what I'm up to, but this is pointless against an AI opponent that sees the entire map all the time. (IIRC, in the original Red Alert, the Soviet player could construct buildings that recreated the shroud of war in areas that the enemy had already explored - and which were totally useless in single player, since the AI was never subject to the shroud of war!) In that case it's not just the player feeling cheated, it actively screws up the player's idea of what exactly would be a good idea against the AI.