The problem being the next latency period, and the next...ideally, they would take blood from you now, store it somehow, then test you in six months and determine that the blood they originally took from you is safe to use. Unfotunately that can't be done - so the only way around it is to allow only ultra-low-risk donors.
The problem with paying for blood donation is that you will attact higher risk donors who are doing it just for the money. Some of these people will, shockingly, not be above lying about their risk status! IV drug use being an example of a high-risk behaviour, and also a behaviour that can make people rather desperate for money. That's an extreme example, but I guess that's why they have to look at not just recent risk factors, but lifetime risk factors.
By only taking ultra-low-risk donors, there's a good chance that I not only test negative for HIV, Hep, etc, right now, but there's also a good chance that I'm actually negative and not about to test positive a bit later on...