Hermosillo is the "owner" of the Mexican clear channel assignment. Canada has secondary rights to 1580. As such, US stations were, in the past, limited to no power or low power at night (Lake Charles, Ft. Lauderdale) or very directional operations such as KDAY. When rules were changed so that KNIX (now the home of Mickey) could go to nights, it was only able to do so by protecting the entire Mexican border.
The "border protection" element applies for both Canadian and Mexican clears (and even the lone Bahamian one) by prohibiting intrusion anywhere along the border. Of course, this resulted in rather chaotic assignments outside the protected nation, as 1580 shows. Both stations there protect the Mexican border by shooting narrow beams out to sea.
Interestingly, although Canada has eliminated 60% or 70% of its AMs and Mexico is in the process of eliminating 85% of theirs, US stations will still have to protect the notified assignments, even if they are not on the air.