I think it's reasonable to assume an additional risk for people in prison.

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.

Not really, contraband includes many things that are completely legal for non prisoners to have like currency, phones, knives, or alcohol. Sending that stuff to prisoners is illegal https://www.law.cornell.edu/uscode/text/18/1791

List of prohibited items: https://www.law.cornell.edu/definitions/uscode.php?width=840...

People working outside pay rent. From a third to upto half their salary.