Just because Greg didn't get a ticket, doesn't make them legal. Tinted lights are illegal. Plain and simple. Headlights, taillights, parking lights...it doesn't matter. It's illegal.
Now, like I said before, as long as you're careful about the resulting light output and visibility and don't run into any dick cops, you'll likely be fine. I don't doubt that you can get by without ever having any trouble from the law for it. But you asked if it was legal or not. The answer is that it's not. So, with that knowledge, you can't very well get upset if you do decide to tint them and one day possibly get busted for it. You were warned.