This is true for me too. My husbands company has about 300 employees. If I had access to health care elsewhere, there would be a hefty surcharge if I chose to be covered by my DHs plan.
My husband works for a national media company with many more employees than that, and this rule has been in effect for years -- long before the Affordable Care Act. An employee who covers a spouse who has group insurance available is subject to being fired.