Why can't they just put them in their checked luggage?
Well, they can of course, but that means people THINKING about that knife they've had in their pocket for the past 47 years, 8 months and 6 days before remembering to put it in their checked bag. And there are PLENTY of people who insist on bringing everything they own onboard the aircraft. You've seen those massive carryons stuffed to the gills, right?

It seems to me that TSA has been trying to deal with a non-issue by allowing these small folding knives onboard. That makes security lines go faster and less confiscation of what very well could be heirlooms.
Actually I applaud TSA for being reactive to public sentiment/pressure.
Now, if the bastards could just STOP GROPING PEOPLE, we might be okay. And those nefarious, evil, bottles of water that people put in their carryons and that get "caught" by the scanner not to mention the stupidity of having to take your shoes off (Richard Reid's still in prison, assholes!) unless you're an ancient geezer, well, we just might be getting somewhere.
A small penknife does not allow for a lot of torque by someone wanting to use it as a weapon, therefore it would not cause a great deal of damage if it were to be used that way. Could it kill someone? Sure. But the assailant would NOT get through the cockpit door with one of those.