They need money to pay for oversight. Any time prisoners talk to someone on the outside, it's a potential conduit for contraband or organized crime.
Even though the enrolled people are completely trustworthy, doing this prevents untrustworthy people to simulate interest in the program just to be able to contact the external world for illegal activities.
List of prohibited items: https://www.law.cornell.edu/definitions/uscode.php?width=840...
Inmates are treated very differently by the legal system than regular people. Thirteenth Amendment: “Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States”
That’s about as direct an accusation as it gets.