You need to prove they did. The only thing the Airlines were concerned with was getting their Gov bailout and making sure the only thing the supposed hijackers had were short knives so that they wouldn't be liable.
http://www.time.com/time/magazine/article/0,9171,1101031201-549004,00.html
Air Support
Monday, Nov. 24, 2003 By SALLY B. DONNELLY/PHOENIX
Remember the airline bailouts following 9/11? Less than a month after the 2001 attacks, Congress rushed through a $15 billion bounty of subsidies and loan guarantees for U.S. carriers — which suffered catastrophic human and business losses that day and the next two. Washington first forked over money for anything that could be linked to 9/11, then paid out $1.5 billion in assistance to five airlines that claimed to be on the verge of extinction.
http://www.cooperativeresearch.org/context.jsp?item=a021104shortknives#a021104shortknives
February 11, 2004: Hijackers Said to Use Short Knives, Not Box Cutters It is reported the 9/11 Commission now believes that the hijackers used short knives instead of box cutters. The New York Observer comments, “Remember the airlines’ first reports, that the whole job was pulled off with box cutters? In fact, investigators for the commission found that box cutters were reported on only one plane [Flight 77]. In any case, box cutters were considered straight razors and were always illegal. Thus the airlines switched their story and produced a snap-open knife of less than four inches at the hearing. This weapon falls conveniently within the aviation-security guidelines pre-9/11.” [New York Observer, 2/11/2004] It was publicly revealed in late 2002 that box cutters were illegal on 9/11.