1) There is a fire/crash axe waiting for him in the airplane's cockpit, so taking the nail clippers away from his was rather pointless.
2) He will be flying the plane, so how could he hijack it from himself?
3) He will be flying the plane, so he could use the plane as a weapon if he wanted to.
OR, most likely
4) He called the luggage screener a stupid jackass for taking away something explicity allowed by the latest FAA security standards...
Oh, joy. I'll go back to being an airline passenger. I can now arrive with properly manicured nails.