A nut with a small explosive (tube of toothpaste) it not the same threat level as some nut with a knife. An explosive just needs to be detonated once to potentially take down the entire plane. A nut with a knife needs to stab his way through the entire plane which despite being one of the "weapons" used on 911 wouldn't happen again with passenger's heightened awareness.

Plus you can build some weapons from items purchased after the security check.

http://www.wired.com/2013/12/terminal-cornucopia/

Airport restaurants only use plastic cutlery when they are located past the security checkpoint.

Edit: US airports. Since this article was about a US airline operating in the United States I thought that went without saying.

At US airports perhaps. It certainly isn't the case in Europe.

The Wetherspoon in LGW only provide butter knives due to security reasons. Not too useful if you order a steak...

Not 100% true - I've received steel cutlery even in an airplane (economy class, if it matters). Non U.S. flight, but it did happen.

I received metal cutlery on a Turkish Airlines ORD-IST flight in coach and a Delta EWR - AMS flight in first. It's definitely not a US restriction either.