All consumer rental car contracts in the US include a provision that disallows the usage of the vehicle for commercial purposes. Additionally, most personal vehicle insurance prohibits the use of the insurance for commercial ride sharing activities without a special waiver.

So as soon as that first fare is picked up (if not sooner), then the renter is in breach of contract. Any insurance from the rental agency is now void, and without special personal insurance waivers, the driver is now driving uninsured.

That is both illegal, and all liability will now be 100% on the driver.

So if someone is going to do that, then they need to calculate in the risk of an accident/arrest and all of the full liability that goes with that.