I think it's because it's cannabis, and cannabis has historically/culturally been a "scheduled"…

I think it's because it's cannabis, and cannabis has historically/culturally been a "scheduled" drug. Doesn't change its efficacy & side effect profile. CBD oil can be profoundly effective if you are in chronic pain, and it doesn't carry risk of life-altering addiction, unlike opioids! Most CBD is derived from hemp, non-marijuana cannabis. Doctors are starting to recommend this to their patients in states where it's legal. Seems effective for anxiety, inflammation, nerve pain, migraine, and possibly mild depression.

America's medication/drug/supplement policy isn't the most coherent.

Look at alcohol - hepatotoxic GABA-A agonist that causes brutal addiction and withdrawals that can kill. And it's regularly abused, causing death by overdose. Yet it's legal. The GABA-A PAMs used to treat mental health & seizure disorders act on the same brain system, and are similarly dangerous (without the liver problems). They're even used for cross-tapering alcohol. Yet these are tightly controlled.

There are a number of other distinctions between "medications" and supplements that don't make sense, as I've learned fighting chronic migraine. It doesn't always have to do with legality as much as quality control/regulation. Butterbur for migraine contains a potentially lethal hepatotoxic constituent if not prepared properly, but there's no regulation whatsoever over the production (it's a supplement). You just have to pick your brand as best you can and hope for the best.