Just because you believe the government won't let the airlines go away doesn't ensure that the bonds are safe. Sometimes keeping a company in business involves a restructuring such that those bonds won't pay out as you're expecting.
Chapter 11 means the business keeps running, but the owners lose their money, Chapter 7 is what most of the world considers to be a bankruptcy / going bust / liquidation. Uncle Sam probably doesn’t care if Delta does a Chapter 11, as long as the planes keep flying.