Even water is a chemical. And not only occurs naturally, but can also be synthesized in a lab, through biological processes, or as a by product of combustion.
Artificial popcorn butter is a mix of flavorings. They may be of natural origin, or they may be synthesized. (usually the latter, it's cheaper these days). You can find the same "chemicals" in real natural butter, good old diacetyl, acetylpropionyl and acetoin. (with diacetyl being the famous one)
You can also make your own popcorn in a microwave instead of buying the rather pricey bags. It helps to use rather fresh kernels to avoid too many unpopped ones.
Interesting popcorn seasonings:
* black pepper and unflavored popcorn salt (it's just fine salt), butter optional
* Parmesan cheese - the cheap kind that is like powder, very light tasting if done without butter.
* a spoon of the powdered cheese from a box of Mac'n'Cheese - optional but super cheesey with butter.
* lawry's season salt (better with butter)
* popcorn salt (with butter flavor) - it brings it pretty close to the microwave popcorn flavor some people really like (but this is edible in my opinion)
* sugar and cinnamon - tends to need butter for the sugar to work (superfine sugar can kind of work without butter)
* sugar, salt and instant coffee - it's weird.