Often airports provide lower landing charges etc for ferry flights (usually 50-75% of full fee) so as soon as one ticket is sold on what was a ferry flight the costs to the airline increase.
It also potentially adds the need for cleaning, steps/airbridge, gates, baggage handling etc. The costs soon mount up for what is an unneccessary inconvenience.
Plus, as has already been said, the cost of the ferry flight is already taken into account in the price of the one-way charter unless it is already company policy to place the empty leg with a broker.