The United States have spent more than 70 billion in funding prevention and treatment of HIV/AIDS outside of their country (mostly in sub-Saharan Africa, e.g. as the Presidents Emergency Plan For AIDS Relief). What do they expect back?
Or maybe, I should rephrase my question: Why is US the only country to spend so much money?
A pragmatic explanation comes to mind: less AIDS in the US. Because people travel to and from the US, occasionally have sex with locals while doing so, and don't always protect themselves.
But for the most part it seems to have been about international PR and having a clearer strategy:
In 1998, while pondering a run for the U.S. presidency, he [G.W. Bush] discussed Africa with Condoleezza Rice, his future secretary of state; she said that, if elected, working more closely with countries on that continent should be a significant part of his foreign policy. She also told him that HIV/AIDS was a central problem in Africa but that the United States was spending only $500 million per year on global AIDS, with the money spread across six federal agencies, without a clear strategy for curbing the epidemic.
To add to Denis de Bernady's excellent answer, it's also the same reason Google spends money on giving people free search, free email, and free WiFi. It's not out of the goodness of their hearts, it's because the more people using Internet, the better off Google's business model (advertizing) works.
Similarly, USA benefits when more people worldwide engage in commerce and spend money and economies everywhere grow (since that growth feeds both US originated investments in those countries, AND US companies' sales to those countries). And the more that happens, the better for both US economy AND US Treasury. (there's many different economic mechanisms involved, but the sum total is that there's benefit to US when economies grow and are healthy and detriment when they shrink and downturned.
And as AIDS clearly has a net detrimental effect on economy (both direct, as people spend money taking care of those who are sick, and indirect, by reducing available actors in the economy), fighting AIDS has an additional benefit of reducing that negative effect.
Think of it as following similar underlying logic to Marshall Plan at the end of WWII (but without "strengthen Western Europe against USSR" component).
The USA is the richest country in the world and as such has a position of Global Leadership. We expect our leaders to do the right thing.
AIDS is a global epidemic. Preventing AIDS in Africa prevents millions of people dying in a horrible way. Preventing AIDS is the right thing to do.
It is possible to be over-cynical. Although there are plenty of times when governments have failed to carry out their duties to the wider world, much of the reason for programmes to prevent AIDS in Africa is that it is the Right Thing to do.
There may be additional benefits to Americans (reduced importation of disease back to America, better Trade, leverage on govenments) or benefits to politicians (Good publicity, seen as generous, kind, and effective, leading to better poll numbers). But a major factor is that the people who make the government act according to their ethics and morality, and not just the bottom line of a spreadsheet.
