They DO work with ICs. But with very small groups of ICs that do all of the shops for a given project. I did over 600 shops for them. They only replace a shopper on a project when she/he retires, moves away from the focus area or dies. In addition, they contract with a couple of MSCs to do a few other shops per year.
However, they also do far more shops that are for employees only. Those require folks with long experience in the catering, hotel or air service industries, for instance.
I did a few hotel mystery shops for them a year or two ago and haven't heard much from them since. The exact same hotels they did in New York were snapped up by BBS and I think there's some relationship between the two companies.