As such, would the US object if I took THC under prescription in an in-patient medical context in another country, and then attempted to enter?
And if they wouldn't object, then why would they suddenly object if I didn't get prescribed the THC, but rather purchased it over the counter, under advisement of a doctor? (After all, this is how most legal, OTC medications work. Doctors in outpatient settings don't prescribe you e.g. aspirin; they just tell you to "take two aspirin and call me if it doesn't feel better." They assume you can get your own aspirin without their help.)