Oh yeah, a good distracted-driving law would cover that, too (sorry!). Our dad used to like to fiddle with the radio when he got going. Of course, he was a slow driver even at top speed so we survived intact.
Modern traffic policing can get too obsessed with things like phones. With modern phones being able to do GPS-like things, which are generally benign, a phone in the hand isn’t necessarily a devil in the car. (Though I found a way to prop it up in the convenience tray.)
How is the car behaving. That’s what should matter.
And likewise, this should encourage cell phone designers to come up with methods of interaction that minimize car misbehavior in traffic. Voice interaction, if done well, is one of those things. You get a text, the phone speaks it or spells it to you. You want to answer, you say your answer, the phone verifies it with you (hopefully with near 100% accuracy) and then sends it. Not utterly distraction proof, but ideally no worse than talking with somebody else in the car.