Not all flights have Air Marshals on board. They don't have the manpower. Also, it's easier to take a hostage with a blade, even a small one. That's not to say I disagree with the decision.
I was referring to the nested article in which air Marshalls (actually groups representing air Marshalls) were "outraged" saying the Marshalls are at greater risk because of these small knives. So, I was referring specifically to flights in which air Mashalls are on board.
http://www.cnn.com/2013/03/06/travel/tsa-carry-on-changes/index.htmlWell, the Marshalls may be a greater risk of attack from someone yielding a SAK, but it's less risk of attack from someone whose preference is pepper spray or a plastic/liquid explosive because TSA can now concentrate their efforts on the latter.