Generally, because they're discriminatory. It's like how racial profiling in law enforcement can be a heuristic that can work. Though, in the case of blood donations, it seems that tests aren't accurate enough to be able to rely solely on them. Also, being discriminated against on being able to donate blood is not as big of a deal. Though in some places with public health systems, stuff like getting a surgery may somewhat depend on you and family/friends donating a certain amount of blood.