I think the intent is to limit the degree with which one can still coordinate nefarious activity outside their prison. (like continuing to run a drug ring from you cell...) And the jailers probably have a good idea of who they want to target. Cell phones might be rampant in jails, but for 90% of the inmates you don't care about. The 10% you don't want to have cell phones have probably gotten much better at hiding them (ie small phones)

All that's just my guess, I'm not really advocating the ban, I'm just trying to think from other points of view.