Making prisoners pay for their prison stay is an appealing idea. I can definitely see why it would be popular with voters. However, you can also see the other side. How saddling people with debt, in addition to a criminal record, can make it even harder for them to fit into society and make ends meet when they get out of jail.
Decisions and policies like this hinge on whether or not our goal for prisoners is punishment or reform. Making prisoners pay for their stay add, perhaps justly, to their punishment. But it makes it much easier for them to reform when they get out, and we already don’t offer them many resources for reform.