Identity Theft From Tossed Airline Boarding Pass?
Sounds like you never boarded a plane pre 9/11. Duh..
Domestic flights in USA had zero security until after the event. You could literally walk to the airport, show your ID once at the check-in (if even then), proceed to walk to the gate, casually flash the boarding pass (no it was not ran through any machines) and board the plane. There were no metal detectors, no security personnell on the lookout, nothing. It was literally like getting on a bus (which nowadays has probably more security than airplanes back then).
Most shockingly, once you had a boarding pass, nobody cared about your id nor did they want to check you further. You could've given the boarding pass to anyone and they could've just walked in and boarded the plane. Of course as with everything, this varied depending on the location, but that was how things worked at the Philadelphia International Airport, among few other ones back then. Had they made an even half assed attempt at running people through metal detectors and x-raying their belongings, chances are that few of them would've gotten caught and rest might have given up.
International flights from USA, however, had standard security. By standard I mean having your baggage x-rayed, walking through a metal detector, etc. However, people were really casual about it and anyone clever enough could have gotten past them as well. So absolutely the hijackers used domestic flights because of this very reason. Chances of getting caught with international flights were just that much greater.
As an interesting parallel in Europe, it was at this very time when European countries were working on abolishing passport checks and talking about even not requiring an ID to get on a plane or having to walk through a metal detector. Not surprisingly those plans were quickly reversed.