The reason I heard that WMU charge apparent UK prices is that when the university set up the flight school they spent many millions of dollars on it (buildings, aircraft, simulators, staff, certification, etc). They want to recoup that money over a reasonable time, and cover costs and make a little profit. Hence the apparent high prices for a US training organisation.
This reason, by the way, was given to me by a WMU rep on their stand at a Flyer Exhibition last year when I asked him the same question.