Well, there's actually a legitimate dormant commerce clause argument to be made here. The commerce clause is the most abused part of the U.S. Constitution, but this is precisely the sort of thing it is supposed to apply to.
-The Constitution divides up powers, granting some to the federal
government, and reserving others for the states or citizens.
-The ability to regulate (that is, tax) interstate commerce is one
granted to the federal government.
-Therefore, the power to regulate interstate commerce is not the
domain of the states.
Now, the "dormant power" argument (the first part) is somewhat debatable, but it is based on one of the very early SCOTUS decisions. It's also what prevents different states from having incompatible vehicle compliance measures, which is definitely a good thing.
This same reasoning has applied to mail-order catalogs (not to mention driving to other states for purchases) for years. States generally try to get around this by instituting a "use tax", but these are pretty well unenforceable in the general case.
In any case, I think we can agree that this is an entirely different matter than the typical uses of the commerce clause, to basically do whatever crazy thing Congress wants to do.
The "dormant commerce clause" power is actually the last bit: B/C the power to regulate interstate commerce is given to Congress, it is not within the domain of the states to interfere with (i.e., by regulating) interstate commerce.
The dormant commerce clause power doesn't prevent states from having incompatible vehicle compliance measures...the commerce clause does b/c Congress has passed legislation on point. States are allowed to have incompatible vehicle standards (Cali and NY/NJ can set their own emissions and fuel efficiency standards) b/c the legislation allows them to seek waiver. If the dormant commerce clause applied, they wouldn't be able to do this b/c there wouldn't be any waivers available b/c Congress was silent on the issue.
I imagine a court will be hearing it quite soon if this legislation takes effect.
And if a case related to this goes all the way up the federal court system, it could get really interesting; Scalia and Thomas don't buy the idea of the dormant commerce clause.
