If there's a glitch in multiplayer then the developer is partly to blame for not sorting it out during testing and for not finding it anyway. If people use it to cheat, however, then they're cheats (obviously) and it's their fault as well. And if they cheat with glitches then they should be banned from the game.
However, glitches in single player are entirely the developers fault, again for not finding them and fixing them. However, if glitches exist in single player, aren't patched, and end up used by players to gain the upper hand, then that's fine. It's in the game, so it should be allowed to be used.