Funnily enough, no reason was given initially, but further questioning seems to link it to a couple of incidents earlier in the year, at least one of which involved a collision with a taxiway sign due to exiting the rwy too fast. The other reason was due to people not being stabilised on approach by 200ft.
Now I had thought that both these things had been countered, firstly by banning use of the first exit off the rwy which is makeable, but if you are too fast you could spin off. Secondly instructors were briefed to ensure students knew what stabilised appoaches were, were able to do them, and if not, were comfortable with go arounds. No incidents have been heard of since, so there is an overwhelming sense of bafflement at why this rule was brought in.
Senior management is due out here shortly, so I imagine he will be approached about this PDQ.