Actually, I can't donate blood due to medications (and possibly also due to being at elevated risk for complications due to blood loss). So yeah, if I'm in a big accident and a lot of people need blood transfusions at once, I'll probably be at the end of the line (I haven't tested this though). I haven't heard of any uproar, though.

Anyway, in a situation where they have to ration blood and to prioritize people, what other way is there to decide? First come first served? The ordering of who came first isn't that precise. In a big accident a bunch of people would come in all at once. I can accept that "he donated blood so he gets it back first" is at least as fair a rule as "he was on the outer ambulance bed, so he came into ER twenty seconds before you, and gets it first".