http://www.wistv.com/story/35833336...a-cell-phone-why-cant-cell-signals-be-blocked
I found the answer to my own question. Apparently, the FCC won't allow the signals to be blocked at the request of lobbyists.
"Former director Ozmint goes further, and says cell phone companies are successfully using lobbying efforts to prevent states from deploying jammers because of fears they might eat into profits.
"Make no mistake," Ozmint said. "This is not anything more than carriers looking at their bottom lines and saying wait a minute, why would I want to stop selling pre-paid minutes that go on anonymous pre-paid phones that are being smuggled into the prison systems by the thousands every day?"