A few years ago? When was this? I've been flying for many years, and I was never aware that anyone and everyone was allowed to carry weapons onto a plane.
How would the airline determine that the guy carrying the gun or knife is a law-abiding citizen? How would they determine that those who don't qualify as 'law-abiding citizens' (e.g. ex-cons or whatever your definition might be) aren't carrying weapons they are not supposed to have?
Until the 1950s sometime, anyone could carry a handgun onboard a commercial aircraft. There was no screening so how would anybody know one way or the other? In fact, some passengers were required to carry them; Those carrying U.S. mail for instance. The requirement started at the beginning of commercial aviation to insure that pilots could defend the mail if their plane were to ever crash.
There were no training or screening requirements. Indeed, pilots were still allowed to carry guns until as recently as 1987. There are no records that any of these pilots (either military or commercial) carrying guns have ever caused any significant problems. Source: P.C. Air Security - When will our pilots be armed? By John R. Lott Jr.
Law enforcement officers were allowed - actually encouraged - to carry guns onboard aircraft until the 70's, I think. FBI and Air Marshalls still carry today.
I believe that it would be easy to "credential" passengers meeting certain requirements and wishing to carry onboard a commercial aircraft with something similar to this proposal: "Airline Safety and Anti-terrorism Act of 2001"