This is the genius of the dollar hegemony tax system. It is voluntary and hidden. No one is forcing foreigners to hold dollars or treasuries.

They do it more or less voluntarily. They could buy gold or other currencies as soon as the dollars arrive, but they choose to hold dollars. This tells me that they think dollars are the best option compared to the alternatives.

Or, more likely, they have ulterior motives such as keeping they own currencies from appreciating. If that is the case they are getting what they deserve when they hold depreciating dollars. Keeping one's currency artificially low is just as evil as depreciating it with the computerized printing press.

The other reason they hold dollars is that the American twin trade and fiscal deficits are so huge that no other store of value is large enough to absorb a conversion without causing instability in that market. The ensuing instability might result in a net loss for any country attempting it.