When ever money is involved the big brothers get involved.
How are income taxes, safety regulations and lawsuits?
Obviously the government is going to view this as a livery business with the appropriate regulations, license fees and taxes. They have to get their cut of the action.
Then you have the insurance companies denying a claim for an accident once they find out that a paying customer was in the vehicle. Once the first few lawsuits from that misunderstanding of coverage are resolved you would see increasing premiums, or a government mandate that you have commercial livery insurance to cover potential losses if an accident happens when a passenger is in the vehicle.
I see ugly written all over this idea.