isn’t it ridiculous that they’d take away a persons ability to be productive while in prison?

If he's in the US, he's probably in a private prison being forced to work for free. So he's definitely being productive in the money machine.

Cynicism aside, prisons in most countries are just meant to punish (it might even be said they're a form of revenge), not rehabilitate, no matter what their propaganda tells you. There's a reason reoffending rates are so high especially in the US, because their prisons basically do nothing to prevent crimes other than instill fear of going to prison.