I think that one at least passable reason for just the “natural” substances to be focused on is to bring religious people under the big tent of eventual psychedelic legalization. Leaning on arguments such as God wouldn’t have made the substance if He didn’t want us to use it resonates with a lot of potential allies to the overall psychedelic legalization cause.
A second reason comes from the group Decriminalize Nature [0] in their resolution to the Oakland City Council when pushing for decriminalization there[1]: “…reestablish humans’ inalienable and direct relationship to nature.” That is, some have this explicit goal and might not care much for the synthesized psychedelics.
Agree with this incremental step from a political perspective as an easy sell / quick win. But, it's imperative that this be understood as an incremental step. The hazard here is equating "natural" with "safe" and "synthetic" with "unsafe".
Eventually, this mindset will have to be dropped because morphine, cocaine, and cyanide are natural (heck, methamphetamines is found in trace amounts in some species of acacia). MDMA is synthetic but can be lethal in a single dose. LSD is semi-synthetic and one of the safest of the bunch--in fact, it's safer than the "natural" chemical feedstocks of its synthesization routes (LSA or ergot).
The shift will have to be to evaluating each molecule on its merits and risk profile. Sure, the whole "natural" argument feels warm-'n'-fuzzy for a lot of people--but sadly it's a poor metric for determining safety. After all, our brains' receptors can't tell whether a molecule originated from a plant or from a lab.
TL;DR: LSD, Cannabis, Psilocybin, Mescaline, DMT, (maybe) MDMA, and their various pro-drugs/analogues should be legal and regulated--regardless of their source. Morphine? I mean, the libertarian in me says "OK", but the more practical side says "probably not a great idea". In the end, none of this is rocket science--but the DEA, the broader US Government, and religion have turned it into a far more complicated thing than it needs to be.
A second reason comes from the group Decriminalize Nature [0] in their resolution to the Oakland City Council when pushing for decriminalization there[1]: “…reestablish humans’ inalienable and direct relationship to nature.” That is, some have this explicit goal and might not care much for the synthesized psychedelics.
0: https://www.decriminalizenature.org/ 1: https://www.decriminalizenature.org/media/attachments/2019/0...