Read the decision again. Checkout where they said Congress “does possess the power to foster and protect interstate commerce, and to take all measures necessary ir appropriate to that end, although intrastate transactions of interstate carriers may thereby be controlled.” There’s your interstate carriers, but there’s also “although.” Seems to me SCOTUS realized interstate carriers could be engaged in intrastate commerce. And such commerce was regulatable not because the carriers were interstate, no, but because they went against the feds’ power to foster and protect interstate commerce. Hence the “substantially related” test.
Here’s the story. The ICC set up a price control regime on interstate rail travel. Certain intrastate lines standing outside the regime “”discriminated” against interstate lines by offering lower prices. This upset the governments power to set prices, so they invented the power to regulate intrastate commerce.
Discriminatory intrastate prices were substantially related to interstate commerce be aide intrastate lines were in competition with interstate lines, not because they were set by interstate carriers. It’s as simple as that.
As for why Wickard did not upturn Shreveport, now that should be clearer. We went from the feds regulating commerce between the states to the feds fostering and protecting interstate commerce, interpreted as fostering and protecting the government’s price controls. From it having to be interstate commerce we went to it having to be commerce that is substantially related to interstate commerce, interpreted as something upsetting interstate price controls.
What was Wickard? An upholding of the AAA, which set marketing quotas and allotments for growing wheat. Which was a means of controlling supply and thereby hopefully buoying prices. A farmer bucked the system by growing his own wheat for his own farm’s consumption. Did SCOTIS misapply Shreveport as precedent in this case because the farmer was not a registered interstate wheat seller? No, because that was not the logic of Shreveport.
The logic was that certain intrastate lines were in competition with interstate commerce. The Wickard decision explicitly argues that the case against Filburn is that homegrown wheat can compete in a sense with commercial wheat. That makes it commerce, and from that point on it tending to upset the interstate wheat price control regime follows the path laid out by Shreveport and others.
So you see it is the fostering principle and the competition principle which unite Wickard and Shreveport. That’s why I say the registered carrier thing is a red herring.
13 Feb. 1829
Letters 4:14--15
For a like reason, I made no reference to the "power to regulate commerce among the several States." I always foresaw that difficulties might be started in relation to that power which could not be fully explained without recurring to views of it, which, however just, might give birth to specious though unsound objections. Being in the same terms with the power over foreign commerce, the same extent, if taken literally, would belong to it. Yet it is very certain that it grew out of the abuse of the power by the importing States in taxing the non-importing, and was intended as a negative and preventive provision against injustice among the States themselves, rather than as a power to be used for the positive purposes of the General Government, in which alone, however, the remedial power could be lodged.
And from Federalist 42:
"The defect of the power in the existing confederacy, to regulate the commerce between its several members, is in the number of these which have been clearly pointed out by experience. To the proofs and remarks which former papers have brought into view on this subject, it may be added, that without this supplemental provision, the great and essential power of regulating foreign commerce, would have been incomplete and ineffectual. A very material object of this power [regulating commerce among the states] was the relief of the states which import and export through other states, from the improper contributions levied on them by the latter. Were these at liberty to regulate the trade between state and state, as much be foreseen, that ways would be found out to load the articles of import and export, during passage through their jurisdiction, with duties which would fall on the makers of the latter, and the consumers of the former. We may be assured, by past experience, that such a practice would be introduced by future contrivance: and both by that and a common knowledge of human affairs, that it would nourish unceasing animosities, and not improbably in serious interruptions of the public tranquility."
The rate schedules at issue in Shreveport appear to be very much the kind of "contrivance" Madison refers to, and effectively imposed an additional cost to interstate commerce. I can find arguable pursuit of the objectives of the original intent of the Commerce Clause in Shreveport. I can find none of it in Wickard.