It's a tax not a penalty, so sayeth the DOJ

Maybe I’m a bit slow, but in regard to healthcare the DOJ is now claiming in their response to the many state lawsuits against this BS that the “penalty” I have to pay if I don't have insurance is a tax and so Obamacare is constitutional under the Fed’s authority to tax. How is it a tax if you only have to pay it if you refuse to buy health insurance? I guess a tax is anything the Fed says it is.