Donating blood plasma is good as it helps people in need. Sure, it sucks that there is a company in the middle making a profit, but not donating is not the solution to that problem, as it hurts the people in need more than the corporation in the middle.
I think its kinda similar to the tipping situation. Yes it sucks that restaurants don’t pay their employees properly and that you have to tip to support the employees. But not tipping hurts the employees rather than the restaurant owner.
In both cases, if we want change, we need to change the legislation.