Spoilers: it's precisely the reason everybody already knows. This article was a waste of time, it might as well say that water is wet.
On the article: I don't go to the movies much at all anymore, but if I do, I throughly enjoy sneaking in my own snacks, or even lunch if it's a long movie in the middle of the day on a weekend. Only suckers pay for the giant popcorn and the barrel-sized HFCS-delivery system the size of an Ajit Pai's coffee mug. Although, I must admit I am a sucker for movie pretzel cheese bites once every few months.