Im gonna go out on a limb and say that, yes, that is their objection. Does that surprise you, or do you find it objectionable? Why? Im legitimately curious.
Maybe because issues in poor nations are straightforward to solve: if you resolve the issue with clean water for a village, that improves lives of hundreds. Issues that plague 'The west' don't have a known solution.
Maybe because of most issues in poorest nations is lack of funs / tech, which he can provide, but cause of suffering in western world is usually political or mismanagement, which he cannot 'provide'
Maybe because for the same resources deployed the choice is between saving 100 kids from disease in Africa vs helping social mobility of 1 kids in US
Maybe you do not believe that from Alaska to Hawaii is 5,000 miles of "we", but 500 miles South from Texas it's "they". Nation-centric worldview is a relatively new idea that only existed for a couple hundred years, even today many people associate by faith, by town/province, by ethnicity or other things over nationality
But more narrowly: Gates Foundation focuses on straightforwardly preventable diseases. These are diseases of extreme poverty that are nearly or entirely absent in the US. The US's problems are of a different nature, largely stemming from inequality in access to healthcare which is a political problem.
It seems rather ghoulish to look at one of the most effective charities in the world, that has no doubt had an impact on millions of people, and say "I want those $$ to stay in the US instead" when we are the most wealthy and powerful nation in history. We need to fix our own political issues, not shame Gates for doing truly good things elsewhere. You want to help the US? Vote for people who want to help the most vulnerable among us vs the most powerful. Our government and society has the resources, they are simply being directed with deliberate inequities.
$4 in america will buy a latte.
Are you, say, a man? Then why would you give anything to a charity that helps women? (Or vice versa.) Or are you perhaps white? Then why would you give anything to a charity that helps people of colour? (Or vice versa.) Etc, etc... Get the point yet? If not, I'll spell it out for you: "Charity" that only benefits copies of yourself isn't really charity.
Like persuading Oxford University to NOT give away the rights to its COVID-19 vaccine: https://khn.org/news/rather-than-give-away-its-covid-vaccine...
Does anyone know if he holds shares in vaccine producing companies?
What would the angle be here? Gates is in the process of trying to give away all least half of his money to charity. His net worth is ~125 billion, is making another billion or two really going to motivate him at this stage in his life?
On the other hand, I've heard that anyone with that much wealth is operating on a completely different mental model than the rest of us. I can only assume he has a very high-level overview of any global aspects that he is interested in and therefore has completely different goals. I suppose that I can't even begin to fathom his thought process and perhaps my confusion is completely unfounded, I just wish I knew the reasoning for his recent Covid-vaccine decisions, which seemingly are detrimental to all human beings on a global scale.
It's been a weird thing for people to turn on Gates and assume he has some diabolical angle. He was an absolute savage in business, and people rightfully feared/hated him when he was at the helm of Microsoft (I was in college during the MS antitrust trial, and a lot of the CS department was rooting for them to be taken down). What I think motivates him now is straightforward self-actualization, trying to undo some of the fairly justified animosity he earned in business for the sake of his legacy. All the money in the world can't buy you a legacy, only accomplishments can do that, and that's what I think he's chasing now.
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.