On most carriers—all of them I think—an employee that accepts a tip can be severely disciplined.

Decades ago, I worked as a flight attendant for TWA. We were told in training that accepting tips can be a firing offense. There was one exception: Charter flights. I flew the New York to Las Vegas junkets often. The charter organizer would pass the hat through the entire cabin, count the money in the galley and divide it equally among the FAs. Somehow a charter flight is a different beast.