AFAIK (have a friends who are Kaiser docs), they are employees, and eventually partners, of the Kaiser Medical Group (a distinct business entity from Kaiser Permanente the nonprofit health system, that contracts exclusively with it), which comes with excellent benefits - including large home loans that are forgiven after several years of service and a generous pension. Perhaps they contract out some hyperspecialist roles.

But neither of those scenarios is really comparable to the ones low wage contract workers find themselves in.