I remember that in my high school time many in my year went to plasma donation as often as it was allowed to collect the compensation. So while you are right that is legally never called payment, people with a need for cash for sure sell their plasma for money.
Oh and in the public sector there is or at least was in the past also the possibility for donating blood and you get the 2 hours or so for that paid as normal. So the government donated the money for a good course.