While I agree that a positioning flight could, in theory, be used to make some revenue for an airline, there is also a chance that with the likes of cityjet, with such small aircraft, may not make that much of a difference.
Passengers mean they have to have an operating crew, and presumably a standby crew should one of the originals not make it. They have to pay for extra fuel, run the risks of delays waiting for baggage/cargo/passengers/airport equipment. They would also need to pay for extra airport facility use at both ends, such as check in desks/machines and baggage hall facilities, gates, ramp vehicles, dispatchers etc. I also wonder if there are insurance cost factors too. It must be cheaper to insure an empty aircraft with some deadhead crew than it is to insure an operating flight. Add on to that the cost of catering for the passengers who, I believe, still get given out for free on cityjet.
I can see why it may just be easier for them to send an empty aircraft to ensure it's at the other end ready for the next flight.