The airline agent consents to passenger boarding the flight when the boarding pass is swiped and the passenger is allowed by the airline employees at the gate to board. An airline may have employee representations or other representations that are not consistent with one another, but somewhere in the process someone with an agency relationship for the airline may well consent to boarding the passenger.

Take the situation of lap child infants on international flights. Sometimes they fly without a ticket even when a ticket is generally required for the booked itinerary and the infant's adult companions make repeated effort to buy the tickets for the lap child infant. Who is an unlawful stowaway in such situation? It is not clear beyond a reasonable doubt that there was intent to be a stowaway. Much the same for someone who may buy concert or sport tickets from an out-of-state ticket scalper for an event where it is claimed that ticket resales are prohibited.

The government making a criminal case out of a civil contract term violations further opens a can of worms that makes just about every one of the age of majority into a potential criminal prosecution target .... all at the whims of the government. Not a good thing IMO.