Their victims may already have been awarded money, and if a prisoner makes money in prison it may end up being used towards paying that down, as any money they would have made.

Earning "okay" money for prison labor is also pretty normal Europe and can give prisoners something to fall back upon once they are released, as well as new skillsets.

If you didn't pay them, prisoners may also end up not working at all. Unlike in the US, slavery there is completely illegal even in prisons ("penal labor"). You can't force them to work by punishing them.

There is nothing to be outraged about, unless you're being blinded by a reprehensible and base thirst for third-party revenge.