There are some underlying differences between most charities and bone marrow donation, though. The registry gets no utility from prospective donors until they match donors with patients, so if they can't reach a prospective donor, that database entry is worthless (except perhaps to artificially inflate the size of the database and attract financial donations). A charity that's already gotten some of your money has already gotten some utility, and would like to have more, but faces less risk from losing contact with people on their list.

Also, can you even sign up for bone marrow donation anonymously? I suppose you could give them fake information but they'd still need to be able to reach you.

I can't think of very many other charities that exist to match non-fungible donations with people who need them, though -- pints of O+ are interchangeable, as are hours of unskilled labor -- so I think you've found a really interesting outlier.