Man boarded Delta flight using photo taken of another passenger's boarding pass

The man man was arrested in Salt Lake City after he snuck on board using a photo taken of another passenger's phone screen while the passenger wasn't looking. He then hid in the lavatory until boarding was complete, but was unable to find a vacant seat afterward. The plane returned to the gate without ever taking off (but it did taxi).

He now faces a felony charge "stowaways on vessels or aircraft."

Sounds like this was just a 26-year-old ski bum trying to get back from Park City for free though - not anyone with deeper ill intent.

Presumably if the flight was completely full, the same boarding pass would have been scanned twice. Why wouldn't the system have flagged this before the boarding doors were closed?