I'm sure he's fundamentally wrong. His logic depends in part on the idea that the "Action B" shares common motivations with Action A. But there's no reason that

I'm sure he's fundamentally wrong. His logic depends in part on the idea that the "Action B" shares common motivations with Action A. But there's no reason that this should be so. For example, thus far the federal government has been unable to pass legislation mandating a federal law for seatbelt usage. We know they're interested because they do try to cause the same end effect, by tying it to State highway funding. Their interest isn't motivated by insurance costs (we know this because thus far the federal government has no direct interest in health insurance costs), but (presumably) by a direct interest in avoiding injuries in accidents. However, if the slipper slope is opened up by allowing the Interstate Commerce Clause to refer to mandating an activity (wearing seatbelts), then it's a fair bet that the federal government will abandon their arm-twisting (viz conditioning based on highway funding) and legislate seatbelt usage nationally. This might be a good thing; that's not the question. The real question is whether taking this first step is likely to precipitate further steps that, today, are understood to be forbidden.