It's part of normal business operations not some secret illegal shady kick backs operation within some kind of spy movie.
There's no need to secretly infiltrate a company when the company is working happily with you. People underestimate how close big business and 3 letter agencies actually are in many cases.
Technically, they collect the company salary, and then the CIA reduces their pay by the same amount. (They have to be indistinguishable from ordinary employee, including to payroll.)
At one point, they couldn't keep any excess if the company pay was higher. Now, they're apparently allowed to keep an amount that doesn't significantly exceed comparable federal pay.
Infiltrated is not necessarily the right word. Company executives know, but few other employees do
And that's not even talking about the less clandestine methods of just ringing their General Counsel and saying "Hey buddy we're the US government we need X Y Z".