In most circumstances I advocate tipping in line with the norm, but when the service is a right and not a privilege, I advocate tipping $0.

If the service is a right, then contributing to a culture in which tipping is expected will disproportionately harm lower-income travelers and foreign travelers who may come from a culture where no tipping exists at all.

There remains the question of subsistence for the employee. A $15 minimum wage is already in effect at some US airports. If you really care about the employees, I recommend supporting broader living-wage legislation, rather than shifting the burden onto travelers who may be no better-off themselves.

Also - to be clear I respect anonymity on this site. But every user's post history is public. Every indication from the post history of Often1 is that he is a man of extraordinary wealth. Two kids flying paid first class; Amex Centurion with waived fees (which must mean he is a VIP customer even among Centurion cardholders). He has admitted to lavish spending on hotel food and beverage. I'm not trying to attack anybody or their wealth, just giving context to advice such as "$10+" tip.