It seems common sense that a GPS unit, even one that cannot be disabled, would only be useful as evidence to follow up on after an alleged crime took place. I don't see any way to use it to prevent a crime happening in the first place.
Look, I'm not trying to be dense here. But the idea that the crime would not have happened if Uber cars had undisableable GPS doesn't seem totally well thought out. After all, "you switched off your phone GPS between these times with someone in the car" is nearly as damning as "you pulled over into a quiet area and the car stopped for a while before it drove onwards".
Additionally, I thought Uber tracks drivers through the customers GPS? That's how they know the car took a longer than expected route? If they relied on the drivers app to self-report it'd only be a matter of time until someone made an app that fed false data to Uber and started selling it to bent cabbies.