Are you claiming that the donors are perfectly rational and have unlimited willpower - and are also perfectly altruistic?

I'm not.

My claim is the modest and surely uncontroversial one that JoshuaZ's "random African" is a better guardian of his own welfare than you, or Bill Gates, or a random do-gooder.

I might agree about myself or “a random do-gooder” (dunno about Gates), but these people do look like they've done their homework.