Actually, there are several exceptions (self-defense is only one) within the law allowing for the excuse of instant emotion to be used. These exceptions may not get you completely off the hook, but they will get you a lesser charge.
We aren't exactly talking about the law, we are talking about social norms. In a social situation with 9 other people, you can't be completely selfish if you value the social interaction. If you are completely selfish, or do things that the other 9 don't like, you may be ostracized or shunned. This game allows someone to act however they want within a chosen social situation, then anonymously and randomly sign up for another. There is no consequence of "nobody will play with me anymore." Those that would like to have a fulfilling social experience find themselves being toyed with by people either being extremely selfish, or just wanting to ruin everyone else's good time. Trolls would not be tolerated within a face-to-face social interaction. I gave an extreme example of a trollish friend of mine, but the consequences can also be less severe; maybe a lack of friends or people you can trust.
You ignore the possibility of the person stopping the trolling. Why would you like to give the troll the power? If someone ran around doing things to piss people off in real life, would we think the problem is the people that are pissed off? Of course not, because that would be stupid.