>you can bring pretty much any prescribed drug in its labelled dispensed container — even the Desoxyn! — with you over the border into the US.

>So it'd be weird if THC was an exception to that

As others in the thread have already mentioned, Desoxyn is schedule 2, while Cannabis is federally schedule 1 (with lower schedule number being more restricted).

So no, THC isn't an exception, it is a part of the same rule system. The one in which schedule 2 drugs have more legal privileges than schedule 1 drugs.