ahh, a sensible question!!
I cannot speak for Terry, guess people will have to ask him themselves at ceremony or other times at the circle.
However, if I may suggest a possible reason....
The reasons for Silbury have long been debated (and will continue to do so), but the general consensus seems to be that it is representaion of the earth mother goddess. It has also been argued that Silbury is a platform for the Celtic festival of Lammas, perhaps derived from the irish custom of gathering on hills to feast at lammas, and the Scottish custom of building mounds at the same time. Add to that that the foundation deposit was made in Late July / early August, then the linking of Silbury and Lammas appear to be quite strong, as does the actual climbing. It would follow therefore that if the hill were safe, then we may all be allowed to feast on the hill at Lammas.
These ideas may be explored in texts such as those by Michael Dames, Ronald Hutton and so on (you know the ones).
Im aware that there may also be quite strong arguments put forward against these ideas, but there can only ever be interpretations - who is to say one truth is better than another. if the intentions are honest and true, then so be it!
As for the fact that this has only been going on for a few years, so what? We all (even Druids?!!) are learning all the time, and paganism is a salient way of thought / religion no? We all try to adopt our ways to optimise the energies and ways of doing, do we not?
Please remember that this is my 'interpretation', not Terrys, yours or the truth!
regards
