In your altered (before it was a race driver?) car rental company analogy, the developer would be the car rental company and Unity the car company? This would mean the developer would rent Unity to its users? Still not making any sense dude.
Apart from analogies. Here are some facts.
A commercial game is a product made by a developer
Unity is a tool that can be used by developer to make commercial games
Unity is also a part of what makes the product work and is shipped with the product.
Unity itself is a commercial product
Take any other kind of commercial product that is shipped along with a commercial product. Is it unfair to charge based upon the number of times that product is shipped?