It'a no more ridiculous a thought than you trying to create an arbitrary separation between me driving a friend across town and someone I don't know.
The separation is as arbitrary as any other taxing criteria. The criteria applied here has nothing to do with driving someone you don't know. It's about charging extra for vehicle registration to companies whose main purpose is to drive around goods or people for a profit. Your original question was "Why should I apply for a commercial license?". The answer was "You don't", because you don't meet the criteria. The discussion here is not about you. It's about uber, which apparently meets the criteria but somehow gets away not paying extra.
since their for-profit use of publicly-funded infrastructure
Which I and my rider pay for regardless of us knowing each other or not.
The question is not wether you pay for the infrastructure but rather how much you pay. You would pay considerably more if the companies that met the above criteria wouldn't have to pay extra. Is the criteria fair? Maybe not. Maybe all vehicles should pay the same regardless of their purpose, or pay based on mileage, but that's a completely different discussion.