US "charity" to the UN became highly qualified as a function of both political distance from the UN goals, and regrettable lapses in probity inside the UN, the kind of problem which crops up anywhere and everywhere. Time and place meant they collided, and the US stopped funding the UN because of <reasons> and instead let the charity sector pick up the burden, which meant right wing christian fundamentalism entered the room. It is little wonder that islamic countries became suspicious of e.g. vaccination drives, made only worse by US polital operatives exploiting the polio vaccination runs to track down Bin Laden.

The US was the most charitable nation in the world but it's not a given.