This material is nothing if not a bit confusing, if only because what seems to be happening diverges from the doctrine articulated by the Court. So let me recap a few basic principles, in hopes of alleviating some of that confusion.
* First, as a doctrinal matter, the analysis is fairly simple. The first question we ask is whether the law at issue discriminates against interstate commerce. If it does, we apply "rigorous scrutiny," which requires a legitimate (i.e., non-protectionist) state interest and that the means be necessary to accomplish that objective. If the court determines the law to be non-discriminatory, we apply the Pike balancing test, and the law is unconstitutional only if its burden on interstate commerce is clearly excessive relative to its putative local benefits.
* Second, a law can qualify as discriminatory against interstate commerce on its face, in its purpose, or in its practical effect. The first two are fairly straightforward, and both clearly identify laws that are intended to discriminate against interstate commerce. The last is problematic category. As Exxon shows, not all laws that affect interstate commerce disproportionately are deemed to "discriminate in practical effect." As I tried to explain last night, I think what is ultimately driving that determination is an intuition that the law is motivated by a discriminatory purpose (even if the Court is unwilling to so state).
* Third, the same sort of thing seems to be going on in the Court's application of the Pike balancing test. That is, it sure seems that when the Court comes to the conclusion that the burdens imposed on interstate commerce are "clearly excessive" in light of what the state is attempting to accomplish, at least a strong inference is raised that something else is afoot. Again, this is not what the Court says in Pike or Kassel, but it seems to be what is going on.
* Finally, let me add one caveat that I forgot to mention last night. There are a class of cases where the Court has found the burden on interstate commerce "clearly excessive" when there probably was no protectionism afoot, and those are cases where states have done something to clog the channels of interstate commerce. Bibb might be the best example. It is unclear how requiring contoured mudflaps could have accomplished a protectionist objective. (I would want to look at the factual record to be sure, but it certainly seems counterintuitive.) Yet the Court invalidated the law. I think the idea is that, given that we are an economic union, states cannot adopt commercial laws that tend to impede the flow of commerce throughout the nation when their ideosyncrisy tends to clog up the flow of commerce. Kassel might also be classified as such a case, but as I said last night, I think Kassel can also be unsderstood as involving protectionism.
I hope this helps in some way. And, as always, feel free to ask more questions.