Someone else's life is on the line. They need to be sure.
Though I must admit, I'm surprised the policy is in place with marrow donations. With blood donations, sure, transfusions are a routine thing and your blood is highly replaceable. With marrow donations, though, someone may need your marrow or they'll die. I'd think a match, even if it carried a slight statistical chance of HIV, would be something they'd want to know about.