I don't usually delve into the TSA and airline policies, since I'm not that well educated on the matter. But, every now and then a story jumps out at me...
Before 9/11 and the TSA airlines were responsible for carry-on screening - the time before airlines charged for checking your suitcase. Also, before everyone started trying to carry-on as much as possible.
The Washington Post had an article about how it's costing the TSA to screen these bags at check points and how the TSA wants more money from a security fee (travelers) to pay for this.
I've finally gotten over the whole pay for your suitcase thing, though I often fly with Jetblue anyway, and now you want me to pay more money for security because I have to pay for my suitcase to go on the plane. If I didn't distrust the TSA then I might not be so annoyed.
but an even more intersting question is... If the airlines were still conducting the security would they be so excited about the money they make off the luggage fees, since they would be the ones forking over the extra money for the extra screening. Well I'm sure they would find a way to make the flyer pay anyway.