If a diabetic is not insulin dependent and in otherwise good health, I don't see the problem with them being able to donate bone marrow. It's not like diabetes is contagious and if the donor has a healthy A1C level, along with good numbers in other bloodwork, I can't see a problem.
But, many organizations won't accept diabetics as blood donors, either. I can understand if the person is on insulin, but my bloodwork looks like a non-diabetic's because my condition is well-controlled. I do take oral medication, but not a sulfonyurea so I don't understand the problem.
I'm not a doctor, though, so maybe I don't grasp all the subtleties involved in donating blood and bone marrow.