When did tailgating become a right?

I don't remember the Constitutional Amendment process, but it must have happened. I ran into another person, and I use that word lightly, that seems to believe he has a right to drive within inches of my back bumper. I was on 7th East about 80th South or so and this fairly new black Mustang gets whips into the lane behind me and thinks that driving close to my bumper will make the person in front of me go faster. So I do the usual, slow down in hopes it really annoys him and he'll change lanes, flash the brakes and at the next stoplight hit the back-up lights. To be honest, I don't know if he saw the brake lights flash or the back up lights, he was really close. It could have been that I stopped a full car lenght from the car in front of me, or maybe he did see the back up lights, but at the stop light he flipped me off. I didn't even flip the bird back at him, I just yelled "Then don't ride my ass". The light turned green, and I went on my merry way, the guy in the other lane was even slower that I was, so Mr. Mustang got around me, but he doesn't just gun it and get away from me. He has to swerve towards me as he passes. I don't even flinch because I'm sure he's a hell of a lot more concerned about wrecking his Mustang than I am of him wrecking my Subaru. But the question in my mind still remains "where do these people get the feeling they have the right to invade my vehicular personal space?"