Most processed food in western diets are unhealthy for you. Usually because the processing basically means very high in sugar, and relatively low nutritionally. It doesn't take much to go from that data point to a rule of thumb "natural = good".
Especially if you combine it with a mistrust of large corporations that you believe prioritize profits over ethics (not exactly a huge leap) - where you think that the stuff they make is bad, but "natural" things they didn't have a hand in cant be corrupted by them.
Real life is much more complex than that, but i can see the chain of logic people use.
Centuries ago sugar was extremely expensive in Korea. Kings would use it as a medicine when they were sick.
It's ok to want something in its most natural form, but you have to make sure that the body is designed to deal with it. Like, don't eat raw potatoes or drink raw milk.
These naturalistic fallacies are certainly a strange, ilogical thing, but one at the core belief of _many_ people. I wonder if it's a western thing though and if we don't find it in other cultures as well.