It's constitutional because it's a tax and the government is allowed to tax its citizens. You can either buy health insurance to fulfill the tax or you can pay the fine to fulfill the tax. Also, as many people have said, you just picked all the good parts of the ACA and said you want to keep those. It doesn't work that way. In order to get those good things, you need healthy people signed up. The only way to get a lot of healthy people signed up is to mandate insurance.