We can't lower the drinking age to 18 even if we think that's sensible because, if we do, then we'll have to lower the drinking age to 10. And we can't possibly do that! As every successful law school student quickly learns, the answer to a slippery slope argument is that, no, logic does not always demand that we slip all the way down the slope. Actually, we can draw a line here or there and then try to defend that line as reasonable or necessary. Slippery slope arguments are not irrebuttable, and if you think they are, you will lose a lot of arguments.
But as law professor Geoffrey Stone explains in this insightful article, in analogizing the Affordable Care Act's individual insurance mandate to the forced purchase of, say, broccoli, some Supreme Court justices seem to think they're on a slope so slippery that there's no way to stop.