>So why does the US allow dual citizenship?
I think that has more to do with the citizenship laws of other countries.
If country X has no problem with a US citizen also holding a citizenship for country X, and that is written country X's citizenship laws, the US has no say in it. The US can't tell another country what their citizenship laws should be.
This was a thing in the Cruz citizenship debate. Canada, at that time, at least, didn't allow dual citizenship. That meant if a Canadian wanted to become a US citizen, they had to give up Canadian citizenship, under Canadian law.
In the reverse scenario, if a US citizen wanted to become a Canadian citizen, and the US allowed dual citizenship, Canadian law couldn't tell a person to give up the citizenship because, under US citizenship laws, they were considered an American citizen. That person would have to voluntarily renounce US citizenship to lose it.
In Cruz's case, he was a minor and couldn't legally renounce his US citizenship birthright.
That's the way I see it, anyway.
IANAL definitely,, but have kids who are dual citizens, so I have had to deal with it. Kids are never asked if they hold other passports when renewing either country. They're not changing citizenship status.