TNT Flight Ops.
There used to be a time when TNT wanted to own all the aircraft used on its routes. The situation of having idle aircraft scattered round Europe all day sat easy with them, because then at least the aircraft would be available for the time-sensitive evening departures into the hub, rather than have some 3rd-party aircraft late back off a charter screw up the schedule. This situation still exists, but will be steadily undermined as further outsiders are let on board.
TNT like to be in OVERALL control of their operation and aircraft, a policy that has served them very well in the past.