Don't you think that the consequences should depend on what the action actually accomplished, rather than the action itself? Flicking a lighter is generally pretty innocuous, but if done to light a house on fire, it means it's a bit different - right?
Yes, it's their fault too for leaving it open, but you had a choice when you decided to access it N000 times, rather than saying "oops, that does something bad, I think I'll stop".
I don't think the punishment fits the crime in this case, but I don't think he's entirely innocent either. Once you've shown that someone stupidly left their door open, the polite thing is to let them know, rather than walking around in their house looking at all their things to show them the error of their ways. IMO a fine would more than suffice as a punishment, though.