I fail to see how this is different than payphones. Even if there was a difference, I fail to see how the appropriate way to stop the behavior of a small subset of people is to punish everyone when you can just punish those individuals. Even if is was the appropriate way, I fail to see why it would apply to prisoners and not the public at large. Maybe we should just turn the internet off for everyone because it's unfeasible to prevent it from being used in a crime?
And pretty soon it's "Send the following codewords to my goons on the outside, or I'll break your knees."