Has anyone ever heard of Bitcoins being used in jail?
Someone could easily have someone create them a bunch of wallets with a sum of coins they have on the outside, then have someone send them the addresses and the amounts. Then trade those addresses. Or, some people have people that visit them all the time and will take care of business stuff for people, and someone could just keep a ledger then have visitation and tell the person on the outside where to send what coins, and who else to contact so that they could get their share paid.
People reading this are like "How can you trust the person to actually pay", well in jail the person usually pays or the person finds some other way to get paid back.
Eventually, there could even be a coin made for this, and eventually petitions could be made for the institutions to start accepting it so that people don't do trading on the outside, unless they are on the outside.
It would also be good work incentive for prisoners if they made this themselves, premined it, and paid them with it. That way people could earn money in jail that they can use outside, as long as they play by the rules.
You obviously have never been arrested and put in to pop, or live in a lahlah land country where you have human rights in jail.