That's pretty strange isn't it? Perhaps the Red Cross views the blood of vaccinated people to be contaminated, and they might have concerns about being sued out of existence.
As far as I can tell from the Red Cross website, they are not blocking vaccinated people from giving blood:
> “Then there’s the American Red Cross refusing to allow COVID19-vaccinated persons to donate blood” <
That’s not strictly true. The Red Cross allows Covid-vaccinated people to donate whole blood and platelets. But not convalescent plasma.
https://www.redcrossblood.org/donate-blood/dlp/coronavirus—covid-19—and-blood-donation.html