Actually, in MMA, boxing and kickboxing, when sparring, you generally go no more than 50 to 70% of force- you're training, not in battle, there's no reason to go full-out unless you're trying to actively injure someone.
I didn't bother mentioning it but Ian makes a great point- that was a cheapshot; any shot delivered from behind could be considered a cheapshot in training, in battle, not so much. Which is why my first comment was that I thought the receiver of said shot should be allowed to feel some kind of way about that. I know I would.