Here is a link to the eligibility requirements from the Red Cross. It is lengthy. I can understand why they don’t want to create an intricate boolean for HIV. See below.
In-Depth Discussion of Variant Creutzfeld-Jacob Disease and Blood Donation:
You are not eligible to donate if:
From January 1, 1980, through December 31, 1996, you spent (visited or lived) a cumulative time of 3 months or more, in the United Kingdom (UK), or
From January 1, 1980, to present, you had a blood transfusion in any country(ies) in the (UK) or France. HIV, AIDS: You should not give blood if you have AIDS or have ever had a positive HIV test, or if you have done something that puts you at risk for becoming infected with HIV.
You are at risk for getting infected if you:
have ever used needles to take drugs, steroids, or anything not prescribed by your doctor
are a male who has had sexual contact with another male, even once, since 1977
have ever taken money, drugs or other payment for sex since 1977
have had sexual contact in the past 12 months with anyone described above
received clotting factor concentrates for a bleeding disorder such as hemophilia
You should not give blood if you have any of the following conditions that can be signs or symptoms of HIV/AIDS
unexplained weight loss (10 pounds or more in less than 2 months)
blue or purple spots in your mouth or skin
white spots or unusual sores in your mouth
lumps in your neck, armpits, or groin, lasting longer than one month
diarrhea that won’t go away
cough that won’t go away and shortness of breath, or
fever higher than 100.5 F lasting more than 10 days.
As for the costs of restricting the donor pool, changes in medical techniques have led to a smaller blood donation industry. Of course that won’t stop spot shortages from appearing from time to time.