Yes, because all humans are perfectly rational and have unlimited willpower.

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

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.

But then again, why doesn't he sell the bednet I gave him and buys beer with it?

Because you gave everyone else in the area a bednet, so now there's a local glut. But yes, I'm sure some people do sell their bednets.