Nothing prevents them from doing pushups, sit-ups, etc. (In fact, several prison gangs do quite regimented calisthenics; I've seen video out of the prisons of this that's downright scary...) And I pay about $40 a month for the gym I belong to.
In other words -- that prisoner's in no different a situation than I am, assuming that your statement is accurate. (I suspect that a more accurate phrasing would be that they need to pay $35 from their Canteen Fund/Prisoner Account, whether they get it by work or outside deposits.)
