If you only have so much money to run say 1000 samples of blood donors, you're simply going to cut out the potentially high risk ones so you don't have to bother with it's costs. Wouldn't you rather test from a batch that will yield 90% good blood than that same batch consisting of a group will significantly lower that yield of good blood?

So with that in mind, they cut out all high risk donors so they don't have to waste money. Whether you're gay, visited Africa, have a disease, have 'at risk' family or had surgery recently, it's simply a method of cutting cots.

If they get to the point where they need a lot more blood, and they are already marketing heavily for donors, perhaps they will open up the pool of acceptance and accept the raised costs of testing all the blood donors. A shame, since the costs will be passed on to the patient… oh wait, I mean the tax payer.