In the US, with the rise of satellite and specialty outpatient services, many (most?) Doctors can often forgo the need to associate with hospitals at all. Several Doctors friends of ours don't practice in hospitals (primary care, allergists, pediatricians), and others are there infrequently (OB-GYN on call).
However, I can't underestimate the power of a coercive government.
At our Ambulatory Surgery Center where the docs buy into partnerships, one of the requiremants is that they must have admitting privileges at one of the local hospitals. Perhaps I was basing my perceptions on that experience.