Does this immunity apply to private contractors, for example at airports that opt for non-TSA screening? If not, that would provide additional protections to the public when private companies do the screening.
Off the top of my head, I'm not positive, so take the following with a huge grain of salt.
But, gut instinct says that immunity applies to the government's agents as much as it does to the government itself. Basically, it's the idea that "an attack on the king's men is an attack on the king himself."
If you're acting as someone's agent, your action is imputed the principal (traditional employer-employee relationship, it's why the employer is liable when the employee injures someone - employer doesn't get the benefit without the burden). In the same way, any privileges that the principal has are imputed to the agent. So, if the principal is protected from suit by law, so is the agent.
At least, so long as the agent is acting within (1) the law granting the privilege and (2) the scope of the agency agreement.
This concept also extends beyond business relationships. For instance, the gov't can't hire a private citizen, have that person break into your house to gather incriminating evidence, and then say that the 4th Amendment isn't implicated because "it wasn't the government that did it." So, with that in mind, if I'm dead wrong on the applicability of the 4th Amendment, the gov't won't be able to get around it by hiring private security companies to do airport security.
I do know that for military contracts (Bell Helicopter, for instance), the contractor gets the benefit of sovereign immunity so long as it complies with what the gov't tells it to do. I think that's known as the
Ferens Doctrine, and I don't see any reason why that wouldn't be extended to non-military-contract settings.