I dunno. Probably not something I would do, and I guess I could see how some people see it as a little creepy... but personally, I have a hard time having a problem with someone bursting another's bubble of a perceived shield of anonymity that protects their actions. I think this is an overall good solution: One person let another person know that their actions are not acceptable, in a non-threatening and peaceful manner, and law enforcement didn't need to get involved (on the off-chance this person isn't who was driving the car, you better believe they'll be passing the email on to whoever it was, and they probably won't be too happy that people they are loaning their car to are pissing people off - it ain't their car that might get keyed). A beautiful example of people working out problems on their own. I think that person is much more likely to think twice about yelling at random strangers now that they'll always have a nagging voice in the back of their mind wondering if they might actually meet/see this person again. And they avoided needing to deal with police. Seems like a win-win to me, for the most part.
It reminds me of one time a friend of mine was riding to work and got honked and yelled at by a driver. Barely a mile up the road, they both turned into the same driveway... where his coworker got out of the car. He saw my friend roll up on his bike right behind him, and went white as a sheet. My friend just gave him a "Yeah, I know what you did" smile, and his very sheepish and now bright red coworker walked inside with him, mumbling some sort of apology.