Because taxpayers pay for the uninsured. Therefore there is nothing constitutional against imposing a tax penalty on the uninsured to pay back the taxpayers.
The conclusion doesn't follow from the premise. The taxpayers pay for everything. There is nothing in the Constitution that says government can coerce people because the government spends money on them. The government pays for grandma's and grandpa's retirement. Does that give the government the authority to force them into a nursing home? Nope.
And my other point was just as valid as manifold's. You can choose to go through life without a car and you can choose to go through life without a job. Be a shareholder. Play the market, etc.
Yes, you can, but when government imposes that "choice" on you, it's not a free choice. It's coercion.
I can hold a gun on you and give a "choice" of handing over all your money or dying. According to you, that's a free choice.
However, that is the liberal conception of freedom: do what the government tells you or die.