All of the taxi drivers I knew in SF when I lived there rented their taxi for the night, up front.

That sounds pretty strong as an indicator they were legitimately contractors - but it's always a complex question. Specifics like: would the drivers be punished or penalized for not showing up for work?, is there an employee conduct agreement the dispatch center enforces and how does it enforce that?, how does a driver qualify for eligibility to rent - can anyone with a cab license freely rent or is there an interview beyond that?

Independent contractor status is a really finicky thing to define and, in the end, it's a status that mostly deprives employees of rights in favor of an employer so any "grey" usage of it should be highly suspect.