If you don't have your own practice, yes, they might fire you. If you're only working "part time", you're bringing in less in patient associated payments for the owners compared to people who work more.

There's also the fact that doctors might prefer $700k/yr working 80 hour weeks vs $350k/yr working 40 hour weeks. A lot of people who become doctors are workaholics and were behaving in a similar manner throughout their schooling.