This is not a fair judgement in this situation. I think money is a far more perverse incentive for donating blood than snacks. Also the snacks are more about helping people increase their blood sugar after giving blood than incentivizing them.
To assume good faith here I'd have to assume wherever you are you've never donated blood?
When you donate blood you've lost a pint of fluids. They give you something to drink to work on rehydrating.
The sugar in the juice and the sugary snacks are to help give you a bit of energy since... you just lost a pint of blood.
Which is to say this is pretty standard everywhere. It's not payment. The ice pack after my vasectomy wasn't "payment for population control". It's healthcare.
The red cross also has their "rewards" program but it's nonsense stuff like water bottles. I think you can exchange points for Amazon gift cards. Like $15/year.
Any downsides?