I believe that's just "friendly advice", not a requirement much less an obligation. Interestingly, while the airline's contract of carriage states that a passenger may be refused transport for not producing positive identification, it says nothing requiring that ID be "government issued". e.g. https://www.southwest.com/assets/pdf...f-carriage.pdf (go to p. 15, par. 6). I've checked in many times at airline ticket counters without being asked for ID, so I don't volunteer to show them any! No problems.