The restrictions came about originally because Scottish Natural Heritage objected to the planning application to build a railway line up Cairn Gorm, arguing that a train would greatly increase the footfall to the summit of Cairn Gorm and beyond, possibly damaging various plants etc. The only way the railway company could get over that objection was by promising that walkers, climbers couldn't access the summit slopes from the top station. This was eventually agreed with the proviso, which came from Europe, that if walkers etc were allowed out at the top the company would have to repay the grant it got from Europe, some £2.6M.
That's the gist of the origins of what appears to be a crazy situation. Whether one agrees with it or not is a different matter. Personally I'd like to see the train removed, but that's unlikely to happen.