IANAL, but this is how it was explained to me:

  -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.