What you're talking about is "waiting to be engaged" versus "engaged to wait."
The drivers are not on set schedules and have no obligation to the company except for the time between accepting a fare and dropping them off. If the drivers were required to return to a staging area and wait for a call the they'd need compensation. But they're not. They can do whatever they want at that point.
When I worked retail I wasn't paid for the time between my shift's end and the next one beginning, but that's what you're arguing for in this case.