If someone needs a bone marrow transplant, their immune system is severely compromised (effectively eliminated). If they're given marrow from an infected donor, they'd have no way to fight off the resulting infection. It would be a death sentence.

On the other hand, many people who need a marrow transplant have some flexibility as to when they can get the donation. This is why the guidelines are so strict: the risk/reward ratio is balanced in favor of caution.