The natural idea implies that there'd be a difference between pure mescaline extracted from a cactus source and pure mescaline from a lab - utter hogwash!
I agree with you that that implication would be wrong. But can you please quote someone making that exact implication? I think you like to argue with strawmen
People who are in to 'natural' don't do extractions of pure mescaline. They are into the chemistry of the entire cactus, such an extraction wouldn't even be 'natural' any more for most of them. Instead, they eat the whole cactus, or make a water extraction/brew in the traditional way. And that's certainly not the same as doing extracted mescaline. You get a cocktail of chemicals, not a single chemical. The exact nature of the cocktail varies from one batch of cactus to another.
I see the 'natural' vs. 'synthetic' argument, as more like eating natural unrefined foods vs. eating processed sugar or other refined foods. When you eat a chemical psychedelic, be it a purified extraction or a purified synthetic, you are eating something highly refined. When you eat a cactus or some mushrooms, you are eating something completely unrefined, and thus a lot less predictable. Many people, including my self, prefer unrefined foods to refined foods. And similarly some people, though not including my self in particular, would just rather prefer unrefined psychedelics to refined psychedelics. Some people prefer herbal drugs to pharmecuetical drugs too when they work. We should respect people's choices. No one is right or wrong on these matters. Its everyone's own body, and everyone's own risk.
Many people who are into 'natural' are also concerned about the ecological wisdom of things they do. When you synthesize drugs in a lab, or do extensive extractions and purifications, you produce a lot of toxic chemical wastes which need to be disposed of. That has a significant ecological impact. OTOH if you just grow cacti or mushrooms, the ecological impact is minimal. So if you really want to be ecologically conscious person WRT to your psychedelic use, you'd stay with unrefined, a.k.a. 'natural' psychedelics. Maybe you think since you're one person your effect on the ecology doesn't matter. Thats not a good thing though when everone thinks that way. So again, we should respect people's choices.
And for some people, growing their own plants just makes them feel closer to the earth. I don't see that as being any different from someone who grows his own vegetable garden. You can say thats a set and setting thing, but a setting is more than your immediate environment, its how you've invested your personal time and energy leading up to the experience. So IOW the only way to create that set and setting is to invest the personal effort. So for those who do grow their own plants, that's their reward for their effort. Again, its a personal choice.
So really, It just comes down to personal preference. I prefer the refined psychedelics my self, because I find them more predictable and I just seem to get more mileage out of the experience. But I can see why some people think its a whole lot more holistic to use unrefined psychedelics, and thats why they do it. I can respect that.