This theory holds that the entire Pokemon Trainer culture (the catching, the battling, gyms, elite four, etc) was facilitated by the manufacturers of PokeBalls in order to facilitate the sale of Pokemon related merchandise.

Before the lives of Pokemon and People became interspersed, there was obviously some tension between the two species. Though not all were harmful, there were certainly cases of Pokemon attacking humans. Needless to say technology to counter this grew and grew, until trapping (or "catching") Pokemon became as simple as throwing a ball at them.

For awhile Pokeballs were simply used for defence, catching dangerous Pokemon. But then people started realizing that these Pokemon could be used for their own devices. The company that made Pokeballs noticed this unconventional use of them, and decided to capitalize on it. They set up "PokeMarts", stores that sold PokeBalls and other items to aid the hunter in his quest for Pokemon.

And, like any other smart business, they sought to expand. So they developed easier products, for the beginner hunter, looking to make the sport more widespread.

You can probably see where this is going. When they realized Pokemon and People could live harmoniously, they saw a great opportunity. It was no longer "hunting", you could catch Pokemon to be your helper, your friend. So they made schools to educate people, funded research labs like Professor Oak's. And, to make sure everyone was constantly catching Pokemon and constantly training them, they built Gyms to encourage competition, and PokeCentres free of charge to aid Trainers in their quest.

This monopoly is why PokeBalls have become the universal symbol for Pokemon. Because the whole idea of Pokemon and People living together was virtually invented by the PokeMart company.

The only downside to all this is the restrictions the company puts on travel. In D/P, you can not visit the battle island until you beat the Elite Four. Which is a brilliant way for the PokeMart company to ensure anyone who wants to travel trains their Pokemon to the highest standard... and it'd be hard to do that without stopping by the PokeMart every once in awhile.

TL;DR: Pokemon Trainer culture was invented by PokeMarts so they could sell a whole lot of product