Fortunately, I'm not very familiar with the prison lifestyle or how currency works but from what I've read in the New York Times, one of the forms of money they use is called JPay which is the payment method used for emails, phone calls, video chats, music, digital downloads, games, etc.

A major investigation kicked off after the Idaho Department of Corrections found multiple inmates had learned how to hack into their JPay accounts and add thousands of dollars.  This money does not affect taxpayers at all, it simply came straight out of JPay and authorities have uncovered at least $225,000 has been stolen by many different inmates.

Jade Trombetta, JPay's spokeswoman had this to say...

While the vast majority of individuals use our secure technology appropriately, we are continually working to improve our products to prevent any attempts at misuse.  In this case, a number of individuals were found to have improperly credited their accounts, creating credits that could be used to purchase content. Once the issue was discovered it was quickly corrected.

JPay has suspended the ability of the inmates to download music and games until they compensate the business for its losses.  Inmates still have the ability to email but that's about it as of now.