the no fly lists prevents terrorists and other dangerous looking people from flying on a plane. if you are too dangerous to fly on a plane, you are too dangerous to have a gun.
the reason they can have a gun, is because due to gun rights they need to be shown to have beyond a reasonable doubt already committed a crime. that's the wrong standard to use, though. it should be that if they are reasonably foreseeably going to commit a crime, they shouldn't be allowed a gun. of course, this could mean we'd have to amend the second amendment, but this would be worth it.
well, it might be possible to use my standard even with the second amendment, but the courts just don't.