Because they love their company. Laugh if you want. I know several F9 folks including pilots and they have an affinity for their airline that many others don't. Remember hearing about the throng of workers who greeted the plane returning with execs from NYC after spurning the Southwest bid and going with Republic? It's a little different at F9 from what I'm told and what I see...
Reminds me of the same kind of spirit that employees had for the original Frontier back in the 80's when all the union groups, except IAM of course, agreed to concession after concession to keep the airline running and eventually came very close to an employee buyout and ownership.