Are the Americans there legally, and not on public dole?
Humm interesting standards Mexico😎
Like so much of human affairs, it's about Pride.
Mexican Pride has been wounded by U.S. deportations.
Mexicans don't care if they're hurting themselves by expelling American retirees and tourists -- and the dollars they bring. Mexican Pride requires a pushback.
Much Israeli/Palestinian intransigence is motivated by Pride on both sides. So too Ukraine/Russia, and China/Taiwan, and India/Pakistan, and African-Americans' behavior, and ...
And well, it's a rare human conflict, foreign or domestic, that isn't at least partially motivated by Pride rather than by rational reasons.