Those airports chose to opt out of direct TSA security screening in favor of the Screening Partnership Program (SSP), which has been around since the TSA was formed in 2001. In essence, the partnership program uses private screeners who perform the same security duties as the federal workforce, though oversight remains with the TSA.
As I said, some think this is a great idea; after all, if customers, meaning passengers, aren't happy with a security officer, a private company could simply get rid of that worker, right? Well, that's a big maybe, but it doesn't matter now because TSA Administrator John Pistole has suspended the program.