How is it Constitutional for the government to punish people for not purchasing health insurance?

If I'm understanding the new Health Care reform right, the government will essentially force people to get insurance. Now, don't get me wrong, I understand the value of insurance, but how can they force it on someone?
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It is a gimmick,,,, the health insurance law with a penalty is just another gimmick to get you to want obamacare.
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