One problem will be if the kite ends up in the water... they'll have to drag it for 5 nautical miles while the ship slows down enough to pick it up again!
I work for a large shipping company here in Fremantle, and if you only knew what we pay a year in bunker charges (fuel) I would be looking for any way to save money.
Having said that the chart shows the ship travelling at 17 knots, which is too slow to be economically viable - you would need to build hundreds of new ships to keep current schedules.
