It's necessary given that the ACA restricts insurers for rejecting people for preexisting conditions. This only really works in a country where everyone has insurance. Otherwise, why not just hang around without insurance until you get really sick, then purchase insurance at that point? If this happens, it drastically distorts the insurance market and makes insurance even more affordable, because people who have insurance will be disproportionately sick.

It may seem like a tax on working people, but really, it is a payment for a very real benefit: a dramatic change in your personal healthcare risk. When you can purchase health insurance at any time even when you're unhealthy, you are essentially being granted an insurance policy for expensive, chronic conditions without even having to purchase anything.