It's not "US copyright", it's several international copyright treaties, which 90% of all nations have agreed to: https://en.m.wikipedia.org/wiki/Berne_Convention

The few exceptions are where copyright essentially doesn't exist at all. Where it does, this is how it works.

As someone above posted the wider "List of parties to international copyright agreements", this is one of the boxes that need to be ticked prior to signing any form of deal between countries. It's a kind of 'fundamental' in order to start doing business with that country (or for the country to be taken seriously).

DPRK only recognises foreign copyrights, and have no concept of IP for its own works (because everything is done to superior order, there is no creativity allowed). Micronesia does not have copyright, but has even stricter regime of creative works, amounting to a patent-like protection.