Starting in 2014, the Affordable Care Act (ACA) requires health plans to cover everyone, including those with pre-existing conditions, and does not allow premiums to vary based on a person's health status. Without requiring everyone to purchase and maintain coverage, there is a powerful incentive for people to wait and purchase insurance only after they get sick or injured. In the 1990's, several states tried insurance market reforms without covering everyone and it failed. Premiums increased dramatically, consumers and employers had fewer coverage choices, and the number of uninsured increased.
Attached is a document providing detail about this ruling.