"I don't think this is a situation where you can say that Congress was avoiding any mention of the tax power. It'd be one thing if Congress explicitly disavowed an exercise of the tax power. But given that it hasn't done so, it seems to me that it's - not only is it fair to read this as an exercise of the tax power, but this court has got an obligation to construe it as an exercise of the tax power if it can be upheld on that basis."