Perhaps these airports are run by the council rather than HIAL and they are nervous about a bunch of old boys messing around in their 1940's planes.
I am a great believer in Hi visibility vests at all times, particularly at a small airport that has just started its first ever scheduled service.
The PPR restriction would be for the twenty minutes when the Highland Airways plane is coming in to land with lots of fee paying passengers. Sounds OK to me.