Says them. But, really, the USA has no ability to force outher countries to follow our laws vis-a-vis former citizens of the USA.
To my knowledge, all member-states of the OECD have "double-taxation avoidance" treaties with each other. That means that if you are a U.S. citizen who is a legal residence of, say, the Czech Republic" and who works there and pays income tax there, you won't have to ALSO pay U.S. income tax (rather, it is offset).
BUT, the disadvantage of that to you, who wants to "go Galt," is that all of these OECD member-states share info about you, the tax-payer, with each other (e.g., when conducting audits).
Bottom line: There is no need for the U.S. to "force" other countries to help "ferret out" so-called "tax cheats." Rather, it's a case of "You scratch my back, and I'll scratch yours."
Regards,
Except there are many countries that don't engage in mutual back scraching agreements. These are known as "tax havens". Different ones have different methods for helping people avoid taxes. It must be working. The carribean tax havens are among the higest holders of US Bonds. (IE: There is A LOT of money in those places.)
And then, of course, just because there is some treaty in place doesn't mean that the jurisdiction in question will actaully live up to them. In many cases they were more or less blackmailed into accepting them, and feel a lot more loyalty to their many private banking clients than to the US diplomat who they are supposed to be cooperating with.
The USA is not the only place were bureacrats can fail to perform to expectations.