Rules in prisons and other high-security places are often extreme. They start with a simple kernel, which may or may not make sense: No personal computing devices (under the assumption that computing devices can be used for un-controlled communication, this may not have been articulated). Someone asks about people using calculators for their GED or whatever, so they permit them. Someone else realizes calculators can be computers (not in just the basic sense of performing computation, but general computers). They extend the rules but keep the carved out exception for non-programmable calculators.