Why are prisoners even allowed to have cell phones? If they didn’t have them, no need to make a law to block them.
They aren't allowed to have them, but they get smuggled in or tossed over the fences. Then the prisoners hide them. The authorities don't always know that a cell phone is hidden somewhere in the prison, and even if they do, it's extremely difficult for them to find it. Sometimes the authorities spend months looking for just one.