You asked a question. You didn't ask me to explain, and frankly, I explained above.

The issue is the car is present because of its commercial nature. Any car on the road is a safety issue, so a car being subsidized by its commercial use which is not properly covered becomes a harm risk to average citizens.

Most "disruptive" businesses tend to do this in some way; translate some form of a traditional business model into a cost structure benefit. AirBNB does the exact same thing. Uber does it not just in insurance and regulatory costs in many markets, but also through price risk.

If the insurance limits are good enough for the general public, why would a driver need more coverage while they are merely driving their Uber vehicle for personal, non-Uber purposes?

Because the vehicle has significantly more wear and tear put on it, and the insurance limits are not good enough for the general public. Commercial use vehicles are always much higher; so you're in essence allowing an anti-competitive exemption for specific people. Additionally, insurance minimums (http://www.cars.com/go/advice/Story.jsp?section=ins&subject=ins_req&story=state-insurance-requirements) are very low because of the societal interest in having people get from point A to point B. When you're using your car to make money, the interest changes to offset the financial incentive to go quickly, not make a repair, etc...

I'm not super worried about the idea of someone using a car to go to brunch and not having commercial insurance, but the normative use case is not that.

It's driving around areas during a shift, nowhere near where you live, waiting for the app to connect you with something. This puts the car on the road for a significant period of time in an uncovered state, exposing the rest of society to harm. You were incorrect above with your assumption that there is a time when they are not working for Uber simply because they have not accepted a fare. They are only present on the road, even for the time driving to and from their house, because of their commercial intent.

The question is, what is the primary use of the vehicle? It's going to be a rare exception where the primary use is not commercial.

Finally, commercial regulation is not simply about insurance. It's about registration for safety inspections etc...