If you're running a co-op game, you can feel free to put a disclaimer of what is accepted and what isn't accepted. It's your game, so its your rules. If people don't want to abide by your rules, then don't let them play in your game. It's really that simple.

As to single player ... there's nothing more that can be said. The majority seem happy with it and that's what matters at the end of the day.