I just looked this up for the Netherlands, and they give medical reasons for not allowing insulin dependent diabetics.
In summary, they need to take a specific amount of blood from someone when they donate. The combination of lower blood volume, potential loss of consciousness and potential low bloodsugar values can be very unsafe for everyone involved. This is why well controlled individuals who are only on tablets can donate, but insulin dependent diabetics cannot.
I don't know how much of this is true and how much of it is covering their behinds legally, but it seems like a logical explanation to me. Maybe in the US they monitor patients during the procedure differently that allows them to donate?