[CRConrad]

Is there any reason to think the money he made and is now giving away came mainly from the US? AFAICT there are a lot more users of Microsoft software outside the USA than within them. With that in mind: Yes, it seems rather ludicrous to demand that he use it for charity exclusively there.

Another reason being that the US is one of the richest countries in the world -- and therefore, logically, among the least in need of charity. (BTW, this shows the objection is ridiculous even if his fortune came mainly from the USA: If he had made his money from selling some luxury article, say Rolls-Royce cars or something, and then decided to spend the money so earned on charity -- should he have been expected to spend it only on helping the kind of people who buy Rolls-Royces?)

Both screamingly obvious reasons IMO, so I find it somewhat surprising that anyone needed to be curious about this.