Huh? Why do you think it is appropriate for the US government to be regulating the behavior of a foreign bank in a foreign country? I have a business account at HSBC in Hong Kong. It stores assets in USD and it is none of your or your government's business. I had assumed the US bank somehow violated American law, because if the only complaints are about the behavior of the Mexican bank, the whole thing smells like a political shakedown.

By your logic the Chinese government should be fining American banks (i.e. Citibank) which offer RMB<-->USD conversion services outside China. Because, after all, the RMB is a controlled currency in China and the behavior of American banks is illegal under Chinese law.