There is an urgent need for plasma, so why not pay for it? Why it is unethical, nobody is forced to donate.
And regarding the risk: When you donate, your first donation is not used, but checked for all known viruses, etc. Only if you are free of any disease, you will make it into the list of donors. And at each consecutive donations, the plasma is first quarantained to wait for some virus checks and only then used. So the risk is quite small. At least that´s how it is done in Austria.