The fact that there appear to be "tolerated" and uninforced instances makes the enforcement of the Chinese-affiliated instance more interesting.But I can think of plenty of instances where enforcement is essentially impossible. If I'm in Buffalo, I can watch most of the Canadian TV networks easily enough over the air. They all carry plenty of US productions, including live sports and late night shows.
I'm assuming that any significant operation trying to use Mexican stations to serve large US markets like McAllen/Brownsville, El Paso, and San Diego / LA would have a "Washington attorney" specialized in FCC issues.