Earlier quoted context omitted.
Inexplicably linking employment with healthcare seems unrelated to at-will employment.
The reason is explicable - health insurance paid by the employer is tax-deductible, while insurance paid by the employee is not. Therefore, employers include it as a way to increase total compensation at a lower cost. The origin of the practice was in WW2, when Roosevelt froze wages. To attract more and better employees, the companies threw in health insurance as a way around the restrictions.
It would be very easy to reverse, if literally anyone was willing to.