The answer, of course, is both: JPay is for prisoners’ use and JPay is using prisoners. This may be a somewhat brusque description of a commercial relationship, but it is a perfectly normal situation. You use the services of your bank and your bank makes money from your business. If the arrangement did not benefit one party or the other, that party would discontinue it. The most important difference between your bank and JPay is that if you are not satisfied with your bank, you are free to take your business elsewhere.
The essential point, which appears to lie behind the question, therefore, is not whether JPay is using prisoners, which it clearly is, but whether it is abusing them. JPay normally operates as a monopoly, meaning that inmates may have no choice but to pay well over the market rate for the services it provides. Some of these are luxuries, such as music downloads, but others, such as access to e-mail and the ability to make basic financial transactions, may be very important or even vital in certain circumstances.
In these situations, JPay literally has a captive customer base. This may even be the case after the inmate leaves prison, since the balance of their trust accounts may be pre-loaded on a JPay card and they may then have to pay a series of fees to access the money. There have been class action lawsuits such as Reyes v. JPay, which claim that JPay routinely takes advantage of inmates’ and former inmates’ lack of bargaining power and inability to seek services elsewhere. These lawsuits are ongoing and most are currently at an early stage, but the sheer volume of complaints suggests that JPay may be abusing what amounts to a monopoly in many areas and charging prisoners highly uncompetitive fees for services which they could access much more cheaply in the open market.