I believe this all dates from just after WW2 when gliding got going again and the CAA did not want to know about gliders. Agreements were reached with the CAA about VFR and VMC for gliders and these rights seem to have survived until the present day.
Most glider pilots know and respect the limits and there have been few real incidents around them. Many glider pilots climb through cloud to contact waves and soaring conditions above the cloud.
I personally think (prepares for incoming
) that this has survived because (CAA) people don't knowhow widespread this practice is and that they don't realise why it's done. However it's only a matter of time before EASA get involved and the associated Grandfather rights disappear faster than cumulus at the end of a good soaring day !!!
BTW - reichmann is probably right, with Gliders that cruise at 130Kts plus they will probably be overtaking you more than getting in your way
cheers
Arc