it gets easier to understand if you adopt the anti-imperialist position, these are vassal states, the us hold incredible influence over them. Gold reserves are just one of thousands of levers at their disposal, if some european nation has a revolution and seeks to assert their sovereignty then the US has a lever to pull. The point to question is the existence of the lever in the first place, the US doesnt have gold in any other country, china or iran doesn't put gold in the US because it gives the US this lever. Thats one of the mechanisms of imperialism, it means if some "western-aligned" (people dont like the word vassal) nation acts in their own interests against the US the US can pull that lever. Why would any nation ever place such a lever in the hand of the US? It's the same exact mechanism why many african nations have the french CFA currency, it gives french the lever to pull its a remanent of colonialism in the case of france.