As we speak, the Supreme Court of the United States(SCOTUS) is reviewing the healthcare law.
They have already indicated they consider it a 'penalty', not a tax, using the argument that most people are expected to get healthcare, avoiding the penalty, so it is not an attempt to raise revenue, but an attempt to punish people. Not set in stone yet, but the tenor of their questions indicate that is a strong possibility.
Next up is the 'legality of regulating inaction vs action'.
The government already regulates inaction. The federal government used to arrest you for failing to register for the draft. The states arrest you for not wearing clothing. The states mandate vaccinations for children - before they enroll in public or private school. As they also have laws forcing you attend school - with some minor exceptions such as home schooling. That is a regulation against inaction, forcing you to act, with some exceptions.
The healthcare laws also have some exceptions - it doesn't force you to buy insurance if you are unemployed, for example.
Using that precedent, if the states can regulate inaction, then so can the federal government. The question then become is health care the territory of the Federal as opposed to state, and honestly that is not challenged (hard to challenge it considering Medicare and Medicaid).
People try to act like healthcare is special - that because it applies to everyone, the government can't regulate inaction. They claim that the scope of the action changes the rules.
But that is irrelevant. The question is not whether the government can force everyone to do something, even if they don't engage in commercial action. No. Wrong. That is TWO questions falsely combined into one, to make it LOOK right. Scope does not change previously established rules. If the government can do it to one person, they can do it to ALL people. There is no rule in the constitution that says the government can do x but only to a small number of people.
The first question is can the government force you to act and second, is the law within the scope of Constitution's mandated federal powers. The answer to both of those separate questions is yes, as proven by previous precedent.
I fully expect SCOTUS to validate the mandate.
This should negate the sever-ability issue (whether the rest of the act can stand if the mandate falls).
Finally, there is the 'coercion' issue - can the Federal government coerce the states to abide by new Medicaid rules by threatening to take away the Federal money they already get.
This is a long standing precedent for this action. The national 55 speed limit laws came (and went) because the federal government tied highway money to it. Similar issues relate to driver licenses rules the federal government wants.
This type of coercion is exactly what the government does. Nothing illegal here.