JR - I agree with your attitude entirely.
As to the original incident in question, I can think of only a couple of reasons why a pilot would be given a potentially unsafe (albeit VFR) clearance - don't forget, as reported this wasn't a 'cleared to descend....' but 'cleared
not above.....'. Whilst those two differents phraseologies have very different meanings in ATC terms, as a (low-experience) pilot may see it, one is an
offer, while one is an
obligation to descend. It is much harder (from a human factors pov) to refuse an instruction than an offer (That said, one of the criteria for holding a pilot's licence must be that one is capable to doing both, where either conflict with safety of the aircraft).
However, there lies the trap for the unwary. Even though all involved are abiding by their respective responsibilities, both in safety and commercial terms (i.e. endeavouring to allow zone access, rather than restricting it), someone has highlighted a way in which the overall level of care provided by ATC (and in the UK, that is exceptionally high to begin with) could be improved -even if it is only in certain limited circumstances, and even if it may result in delays for those wishing to transit CAS in and around terrain.
How about
: "G-CD, are you able to remain clear of terrain below 2000' on the RPS?" first, before the clearance is given to that effect? Or something similar.
That may take up a bit more RT, but so does "Negative Manchester, cannot accept that clearance due terrain, request blah blah"
Even if it just makes ATCOs think about the implict safety implications of imposing altitude restrictions in the region of high ground, Jump Complete makes an important comment - one I might humbly suggest is exactly the kind of thing
CHIRP was invented for.