Huh, interesting to know! I can see the reasons for that rule pertaining to any medical donation generally, but in this case it does feel like a clear reason for an exception. (In my non-medical-ethics-trained mind.)
If my blood was withheld from people that needed it and instead destroyed, only BECAUSE I had a disease that benefits from bloodletting and does not affect the usefulness of my blood when transfused in any way... I think I would be rightfully angry by that.
The point isn't to cause you to be angry like that; that is an unfortunate side effect.
I believe the point is to stop predatory pressure to sell parts of your body because you need money or something and someone with more money or power than you wants it. We have to make laws like this to prevent some people from abusing inequality to create systems that put people in a position where they need to decide if they should sell some part of themselves.
In a perfect world people should be able to make clear, informed decisions about what to do with their body. In this world we have to set some baselines to prevent exploitation and permanent harm because we know SOMEONE will do it otherwise.
As an extreme example, imagine if, instead of stealing some money through gift cards, scammers got grandma to give up one of her kidneys.
If my blood was withheld from people that needed it and instead destroyed, only BECAUSE I had a disease that benefits from bloodletting and does not affect the usefulness of my blood when transfused in any way... I think I would be rightfully angry by that.