For good reason. Uncontrolled inmate communications result in things like murder:
"The complaint alleges that Corbett planned several killings with Ronald Dean Yandell, aka Renegade, an Aryan Brotherhood leader who prosecutors say sits on the three-man commission that runs the gang. Corbett and Yandell were incarcerated 220 miles apart from one another, but regularly communicated through contraband prison cell phones, prosecutors say." https://www.mercurynews.com/2020/02/01/ca-inmate-was-killed-...
"Consider the case of a, a 38-year-old Maryland resident who had the bad luck to witness a street murder in Baltimore and the rare courage to agree to testify against the accused killer, Patrick Byers. According to prosecutors, Byers acquired a phone while awaiting trial in Baltimore's City Detention Center. He obtained Lackl's name, address, and phone number and allegedly texted that information to a friend on the outside, along with an offer of $2,500 to get rid of Lackl. On July 2, 2007, the friend rounded up a couple of thugs and drove out to Lackl's modest suburban home, where authorities say the crew blasted him to death with a .44 Magnum." https://www.wired.com/2009/05/ff-prisonphones/
"While authorities say Johnson is the first corrections officer in the U.S. harmed by a hit ordered from inmate’s cell phone, other people have been targets. In 2005, a New Jersey inmate serving time for shooting at two police officers used a smuggled phone to order a fatal attack on his girlfriend, who had given authorities information leading to his arrest." https://newsone.com/753345/prisoner-ordered-hit-outside-of-p...
There are more examples, but I think these illustrate that it's a very good idea to have control over inmate communications.