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You suggested it was for navigation in your first post. Do you really think a black cab driver is using a tablet on the passenger seat for navigation? Or do you think they're using it to either accept internet fares or catch up on Game of Thrones while waiting in a taxi rank? You've not actually said they were using it while driving so I'm guessing they weren't or it would've been mentioned.
Sure, but I'd argue it's pretty distracting looking at a big fuck off screen on your passenger seat.