It's only a fallacy in the same way that the slippery slope is a fallacy, ie when used as a logical basis in and of itself. There's nothing wrong with either one when used appropriately in context.

I mean, of course, there's a spectrum when it comes to eating food that is "natural". On one extreme side, you have beating wild pigs and drinking from udders. On the other extreme, you have... whatever the fuck is in Doritos and soda pop. Part of figuring out your own dietary practices is finding the right spot along that spectrum, to at least attempt to draw that line yourself.

Yes, a lot of people who simply complain about vague "chemicals" in food are nutters, but I don't think that means that I'd be OK with eating every mass-produced-food-like-product offered for sale.