Instead, companies could set reasonable standards for productivity and discipline those who don't meet them. It doesn't matter if an unproductive employee is unproductive because they are on their phone, or because they spend too much time at the watercooler, or because they are just pretty bad at their jobs.
I think part of it is a (work) cultural problem.
At one employer, working in a sort of repair/assembly atmosphere, my boss text messaged while I was working. In fact, he sent several text messages.
Hours later he asked me, "Hey, I wanted to talk to you, didn't you get my text messages?"
I told him that I usually don't check my messages while I'm working, and that I could do that if it was what he wanted.
I didn't get the feeling he was testing me; I genuinely think he expected me to stop what I was doing, pick up my phone, and read my messages as I received them. I don't think he realizes that it means every time I get a text message, I have to stop what I'm doing and take out my phone to verify if it is from him. That from that point forward, every text I get is money out of my employers pocket.