For a trade to be fair, it has to be fair in both directions. The source producers need to be paid a fair price for the product the produce, but I too need to pay a fair price for the item I’m purchasing.
It’s called Pythagoras Theory because it’s only a theory. Every triangle that it’s ever been tested on works, so there’s no reason to doubt that it will continue to work on every triangle we ever find. But until there’s a way to prove it, it will always be called a theory. If it’s ever proven, it will be called Pythagoras Rule.