While waiting for my MSY-DFW flight yesterday afternoon (AA1255), the previous flight from that gate was about to complete boarding when a girl was escorted off the plane by an FA. The girl had managed to sneak by the gate boarding agent, but the FA discovered she didn't have a valid boarding pass for the flight. I would guess that the FA noticed a discrepancy in passenger count and the manifest. The girl apparently had a boarding pass of some type, at least that's what I overheard, as evidenced by her ability to get through TSA screening. The FA escorted the girl to the gate counter where she was allowed to use her credit card to purchase a ticket for the flight. She then boarded and the flight departed. Moments later, a dozen TSAs checked the boarding pass and ID of everyone in the waiting area.

My question - why was the girl allowed to buy a ticket instead of being arrested?