Do you remember all of the stink a year or two ago when SWA "lost" to kids who were traveling alone and were NOT UMs? They got off the airplane at a through stop (MCI) to get something to eat but didn't re-board on time? Unless I'm confusing incidents, there flight was the last one of the day and SWA actually called the airplane back to pick them up.

The airline knows that it will be held responsible for anything that might happen even if it doesn't know that it's carrying the UMs.