Paying for bone marrow became illegal under the ban on organ and tissue sale, which is understandable. It is likely that bone marrow is not treated like blood in this equation because bone marrow donation used to be a much more involved procedure with a recovery period, and part of the idea is to avoid people donating for the money without regard for the risk. Since most bone marrow donations these days aren't much different from donating blood, that reason starts to fall flat. OTOH, one could argue that a campaign to make people aware of the PBSC donation method might increase the number of people willing to donate as well.
Like Dagrimsta1 said, opening the possibility of sale could end up introducing a competitive market for blood marrow wherein wealthier people are at an advantage and poorer people are essentially denied marrow transplants. While we can say that it
should be carefully set up and/or subsidized so that poor folks can afford it, the reality is that the people writing our laws are mostly wealthy, and it is a disadvantage to them & their families to avoid a free market situation. Subsidization, like jd989898 said, is an interesting proposal, but unfortunately people are sometimes outraged that those who use subsidization can have nice things, like fresh produce (or, say, healthy donated organs/tissues), and then budgets or restrictions on entering the subsidization program get tightened.
It's not a simple issue, even though it feels like it should be
As long as you know what bone marrow donation is and know that organ/tissue sales are illegal, I think you could talk your way through it.
I guess the point is probably to read up on a variety of ethical issues
Edited for correction of repetitive phrasing.