The skycaps are the exception. They are not airline employees and are paid wages that assume they'll get tips (ie below minimum wage). Or at least were in the past; I can't keep up because I never use curb-side checkin. (And there's a separate saga about AA collecting a curb-side check-in fee which customers thought was a tip but didn't actually go to the skycap.)

The reality is that, with American (the country, not the airline) tipping culture, consumers are generally expected to somehow know which customer-facing workers are professional employees (therefore salaried and not expecting/able to accept tips) and who is a contractor or other employee (who is expecting tips and often paid a lower wage as a consequence), essentially guessing for ourselves without any real information what an appropriate wage for a given level of service is. But American Airlines employees, including FAs (not to be confused with some contractors, but FAs working for contract airlines are like AA-employed FAs) cannot accept tips. Whether they're paid decent wages is another topic.