I'm going to take a shot at this, with apologies in advance to a certain UA guru if I'm off a bit.

In UA land, premium class and status pax on short-haul international (which encompasses Canada/Caribbean/Mexico) flights get lounge access but don't get any free drink tickets in the RCC. Adding to that I think Mexico City is treated differently, that is as a full-blown international flight with all the perks that go with it; don't ask me why.

Your average RCC guard knows at least the basics of this policy. Don't tell anyone, but I've even gotten a few drink tickets on trips to the islands a few times.