The answer is "because." Really
Standby is not allowed for international flights. According to the AA definition (used elsewhere as well), Mexico is considered domestic (hence standby allowed),
except Mexico City, which is international (hence no standby allowed).
See, I told you that the reason was simply "because."
Cheers.
I believe MEX has the same odd exemption in regards to usage rights to the Admirals Club for PLATS and EXPLATS. For everywhere else you fly to in Mexico, you are not allowed status based access to Admirals Club (just like domestic flights)