So long as it’s not coercive (they aren’t getting punished or having their sentences elongated for not donations) there is nothing ethically dubious about it. Who cares they are getting something in return? Do we expect the farmer to grow food simply because it’s morally good to feed people or the doctor to forgo payment for services because it’s the right thing to do?

The world would be better if we dropped the insane prudishness and high minded moralizing around organ and marrow donation. The USA is the source of 70% of the world’s blood plasma, because we allow people to be paid for it. Allowing people to get paid for doing something good, whether in money or reduce prison sentences, is morally fine. It generates new organs and marrow that will save peoples lives, lives which otherwise probably wouldn’t be saved. So then how can you say that’s bad?

In fact, we should allow markets for kidney, liver, and marrow - all are things that can be donated safely without significant long-term problems for the donater, and will save peoples lives (and also money by cutting down on the amount of time people are waiting for a donation).