Its a common scene where a person will come out of a pub and debate which is the quickest way into town. Maybe by bus as it may be cheaper but then you have to wait for the bus and stop at stops along the way. They could get a taxi and have a direct run to your destination.
The person decided to order a cab but whilst the cab is on its way to pick you up, the bus arrives so you jump on that and head off into town and cant be bothered to ring the taxi off to cancel the cab
Its a common scene and happens a lot locally, usually where there is a bus stop near to a pub or other venue.
It only takes a minute to cancel a taxi but they cant be bothered. The other thing is that people will wait for you to pull up outside their house before cancelling. Should they be made to pay a cancellation fee. After all, would you order a pizza delivery and wait for it to arrive then ring up and cancel the order.