We don't know the causality so its better to play it safe. What if pre-dementia or pre-alzheimers donors have the protien and it really is transmissible? For something as lethal as either of those diseases you would err on the side of caution and refuse donated blood with those proteins.

A massive contributor to why HIV/AIDS is so bad in Africa is becausecof pooled blood products that were contaminated with HIV from donors even after we knew it was transmissible by blood... those companies packaged and sold or donated those products to Africa seeding the current crisis.