This reminds me of "The Copenhagen Interpretation of Ethics": http://web.archive.org/web/20160806191858/https://blog.jaibo... Our culture has a very strange belief that you shouldn't benefit from the less fortunate. Even if you benefit them too. People that have never donated a dollar to charity in their lives, will come out of the woodwork and complain about "exploitation".
"People that have never donated a dollar to charity in their lives" is a particularly American, or let's say anglophone, way to approach social justice. Charities are based on good will, but "the less fortunate" do not need "good will": they need social rights. https://en.wikipedia.org/wiki/Social_rights If you don't see "exploitation" when people need and they are willing to sell their blood, I have no idea what you…
As a father this situation would be horrible if my only chance to provide things for my daughter was taken away to "help" me:
"he was giving plasma because his daughter had a birthday, and he needed $7.50 to buy her a bathing suit and money for a cake"
Sure, you can say it would be better if the dad had some other opportunity available, but that's changing the subject. This is what he has right now, that gives him more happiness than anything in his life.