The Shreveport case involved regulation of intrastate commerce indirectly because it was done by a registered carrier of interstate commerce. The decision states "by reason of their control of the carriers". What was being regulated was the carrier, as an instrument of interstate commerce. By robertpaulsen's logic, it was necessary and proper for Congress to declare each and every one of us an "instrument of interstate commerce" for the purpose of regulation under the Commerce Clause.
I wasnt saying I bought his application of it, just supplying a possible source of his logic.
"by reason of their control of the carriers" was the necessary nexus which allowed the court to rule as it did.
The court was saying that because there was a connection, a relationship, between the interstate and intrastate rates by a carrier, Congress could step in a regulate the overall rates. The predecessor to "substantial effects" if you will (the court used "substantial relation").