Don't they test donated blood for disease in the U.S.?

To an extent, of course they do. But a full battery of tests is very expensive, so they're more likely to do the less expensive screens, and even with the more thorough testing, some disease can still slip through. This is why they don't want you donating if you just came back from someplace that has endemic malaria. It's not just gays who are asked not to donate; there's a whole raft of questions they ask you to determine the chances you might have a blood-borne disease. They try to dissuade people from donating who have a high risk of having infected blood. And in this country at this time, unfortunately, that includes gay men as a group.

Anybody who is interested the AIDS epidemic could do a lot worse than reading And the Band Played On by Randy Shilts, a very thorough history of its early years and the original epidemiology and subsequent lab science that discovered its etiology and modes of transmission.