A few facts - three people in the UK have died of vCJD (the human form of BSE/Mad Cow Disease) because they were infected from a blood transfusion. http://www.cjd.ed.ac.uk/cjdq60.pdf
The important thing about this is that the person who gave the infectious blood appeared to be perfectly healthy at the time they gave the blood. So we know people may be infectious a few years before they develop symptoms.
Three Americans have delevloped vCJD after spending time in countries that had BSE (UK and Saudi Arabia). http://www.cdc.gov/ncidod/dvrd/vcjd/qa.h...
One of them had lived in the UK for many years, moved to the USA in 1992 and didn't develop symptoms until 2001.
One other thing - in animals it has been shown that an animal can be infectious without ever showing symptoms.
So basically I think the USA is being extremely cautious because there can be very long incubation periods and we have no way of knowing how many people could be infected but never show symptoms. They probably they will continue to be cautious unless there is a real blood shortage. However there have been cases of vCJD in other countries, not just the UK (e.g. 23 cases in France), so I suppose if the USA was being obsessively cautious they could ban blood from those places too. http://www.cjd.ed.ac.uk/vcjdworld.htm
In the UK, we're not as cautious as the USA, since everyone has already been exposed to BSE in food. Here, you can donate blood that is only used for other UK people. However it is specially treated - all the white cells are removed, which is thought to greatly reduce the chance of infection. Products containing white cells and plasma products are all sourced from abroad. Anyone born after 1996 does not receive any UK blood products. Also in the UK you cannot give blood if you have received a blood transfusion yourself since 1980.