The move to Mexico was just a temporary move to stay polygamous, they are and were Americans, they came to serve Joseph Smith 170 years ago and have never stopped.
Yes, it turned out to be temporary because Mexican anti-communists ran them out of the country. Those Mexicans had no tolerance for “polygamous communes.” The Romneys and all of their “wives” were forced to flee to the United States. We’d be a lot better off if the Mexicans had just thrown the whole bunch of them into a Mexican prison.