I think FDA rules explicitly prohibit paying blood donors in the US. Ostensibly because if you do, the donation centers fill up with junkies who'll lie about not having hepatitis so they can get paid, or steal IDs so they can go twice a week until they die of anemia, raising the costs of safety testing and generally being injurious to public health. Of course, everyone else involved in the process gets paid, just not the donors -- quite dearly, as you'll learn if you're ever on the receiving end of a transfusion.
Plasma is paid because it falls under a different regulation and their research and industrial customers don't care that the plasma came from a crack whore.
Sometimes it means "We don't want to spend a lot of money training this guy who we won't be able to retain if he gets a better offer."
At really entry-level jobs like fast food, where training is quick and turnover is always high, it sometimes also means "This guy might be able to read the workers' rights poster on the door and explain the workers' comp program to the idiot who spilled boiling grease on his foot.