The hospitals don't discriminate by income. They will be happy to take blood from anyone.

The economic incentive for poor people to donate blood is much greater than someone rich, therefore more poor people donate because it benefits them more. 80 dollars means much more to a part time minimum wage employee than a CEO of a international corporation.

Besides, what is wrong with a poor person donating blood a few times a year to make an extra few hundred dollars to help make ends meet? He benefits by increasing his income, and the hospitals benefit by having more blood available for emergencies.