Neal,
Sorry to hear you had issues donating blood. The requirements are pretty clear cut and specifically say on the forms they make you read (although, I have to admit, I don't always read them...) that you can't donate if you've had a dura mater transplant. Since you weren't sure, I'd say they made the right call in asking you not to donate at that point. They're just following the guidelines, and if I were ever to be a recipient of blood, I'd want to know they weren't bending the rules along the way.
I checked with my doctor and the Red Cross after my surgery to make sure I'd be okay to donate and was told by both that it would be fine, as long as there was no transplant involved. I was also able to remain on the National Bone Marrow registry after my surgery.
I think the biggest risk after having a dura mater transplant lies in transmitting Creutzfeldt- Jakob disease (I'm not positive if this is the reason for excluding those who have had transplants, but I'm pretty sure this has at least something to do with it). CJD will cause a slow and very painful death - not something you'd want to inadvertently pass along to someone you're attempting to help.
The Red Cross might be picky sometimes, but we should probably be glad they are - you never know when you might be the one needing the blood - and you're not going to want a bad batch.
Donating blood is, in my opinion, a great way to give something back to those in need. Please don't let this one bad experience ruin that for you.
Lori