How long are you taking to read a text message or respond to it? Something tells me that it takes more than one second to look at the screen and read a text, and I'm pretty darn certain it takes more than a few seconds to compose a text. Changing a radio station can easily be done without taking your eyes off the road, or only doing so for half of a second.
And, I've always loved the "reading a billboard" part. You are right, you shouldn't spend time reading billboards. But there is one huge difference. When looking at a phone screen, your focal point is no more than two feet from your eyes. And if you're looking at it down in your lap or on your car's console, it's completely out of your view of the road. Your eyes looking at something two feet from your eyes is very different from looking at something 100 feet or more away. Looking at the billboard doesn't change your focal length, and generally keeps you looking only a few degrees off your intended view point. Quite different from looking at a phone screen.
Now, I'd love to see billboards all gone. I hate incessant advertising, and massive displays blocking the natural view.