As the kids say, "Dude, this is America." Why would merely having a prescription and a reasonable excuse help you with the border patrol? Maybe you'd get a sympathetic guy that day and maybe you wouldn't.
>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.
Plenty of substances are dual scheduled, desoxyn is just most prominent example.
Are you sure you're not getting it mixed up with MDMA or something?
0. https://www.justice.gov/archive/ndic/pubs5/5049/5049p.pdf