I used to wonder about curbside skycaps ask for tips but inside check-in agents never ask for tips. A friend used to work for an airline told me that curbside skycaps are contract and not airline employees, where agents at check-in counter inside of the terminal are airline employees.

What I do not like is inconstancy between tipping outside at curbside and no tipping inside curbside. As far as passengers (consumers) are concerned we are getting same service. This is like if I sit at one part of a restaurant then no tipping to server but if I sit at another part of the restaurant then tipping is expected. I ordered same item and received same service.