While we may wish for more airports to do this, don't. The airports would still have to follow every random TSA mandate regarding security, except now instead of being annoyed at the TSA for the rule people will start focusing their anger at half a dozen secondary security companies instead of at the government issuing the mandates. At that point the TSA will be free to throw out whatever mandates it wants without worrying about facing any sort of public backlash. It would even encourage them to issue crazier mandates faster and faster as they wouldn't be the ones shouldering the burden of buying the equipment (but they would be mandating specific equipment from pre-approved vendors, of course), training the operators, or anything else involved in what we like to call reality.
Two more things: Do you really think these half a dozen pre-approved security companies didn't have to throw in some major kickbacks to get that approval? And secondly, another potential word for security company? Mercenaries.