Your EYES are on the road, but your FOCUS is divided between driving and telling Siri what message you want to send. Having eyes pointing one direction doesn't necessarily mean you are seeing what's in front of you.
That said, I don't drive myself so I don't know how much talking to Siri detracts from driving. But there is SOME level of distraction there, even if your eyes stay on the road.
Really no different from anything you do hands-free, including carrying a conversation with someone over Bluetooth or even with the passenger sitting next to you. Maybe pass a law banning anyone in the car from talking to you