That seems like the default I would expect from imprisonment.

In a libertarian framework, the function of imprisonment is to force the prisoner to repay their debts. Denying the prisoner access to any value creating opportunities stands in the way of that goal. If a friend or a charity wants to gift something like books or basketballs to the prisoner, I don't see any justification for superseding the prisoner's ownership of those things.

In a statist framework, the function of imprisonment is to inflict punishment on the prisoner. Since suffering is the intended purpose, I don't want to give the government any good ideas on how to maximize it.