Why gliding sites do not generally welcome power visitors
It is primarily a SAFETY MATTER! A previous poster has very carefully listed the points that make a gliding site quite quite different from a power site. Booker has co-existed with 2 power clubs and a helicopter operation, but I wouldn't say it has always been a happy relationship. Typical; below the LHR zone and the circuit height at Booker has always been a chosen path for light avaiation, which does not always mix well with gliders thermalling over the airfield.
At Shenington, which is my home club, three times while towing up a glider I have had to take evasive action from a power plane which simply was NOT looking out; and once went so far as to file an airmiss.
But the most horrendous of all was being on the ground and watching a self-righteous helicopter pilot who considered that phoning the week before = unfortunately no record was made of this call at the club - and calling on the day on an airfield frequency - which typically of a gliding club was not manned; and then took off directly in front of a glider (non radio) on approach after the glider pilot had just completed his first 5 hour endurance flight.
On another occasion, a visitor from a power club who had been briefed about the hazards presented by winch cables, took off (we suspect after a liquid lunch) and flew into the cable! which slid off his wing, and he carried on to his destination!
It is our frequent experience that power pilots fly in straight lines, mostly, and seldom maintain a decent lookout. And all too often fly straight across an active gliding site.
For all the reasons before cited, we prefer not to be visited by power aircraft who are unacquainted with gliding. We welcome any pilot who would like to participate in gliding, but far far better if you come by road, please.