I find this a very difficult question to answer and the opinions interesting.
Reading the original post again I note:
No problem, says the pilot, I'll take a VFR transit. Ok, replies the controller, descend to 2000', report VMC.
Presumably the aircraft was already in IMC (given what FFF has to say about the met) outside CAS. That leaves me wondering whether the pilot declared he was IFR on his first call. If he did declare IFR, presumably he was responsible for his own terrain seperation, but could he have made the descent given that he never became VMC at 2000ft. This seems the more likely scenario because the controller said "report VMC" implying he knew he was not in VMC at the time of the first call. If he declared VFR on his first call then presumably, and not unreasonably, the controller assumed he could descend and remain VMC for his transit but again given what FFF has to tell us about the met the pilot was not telling the truth.
Having made his descent, whether from IMC or VMC, again on the basis of what FFF has to tell us, the pilot presumably found he never became VMC. Presumably had he told the controller this was the case the controller might have been a whole lot more receptive to an IFR transit and a climb to SSA.
Subject to what actually happened it seems to me the pilots "sins" may have started even earlier.
For these reason I have a feeling Rustle is correct.
Too many pilots these days it seems to me are prepared to declare VFR when they are not. Clearly some of these will cause problems for themselves (as I am sure there are more than a few without an IR) never mind problems for others. At least by CHIRPing, if the full exchange is known, it might reveal how a series of events could lead to a very real problem, and, more importantly, how these problems could have been avoided in the first place by the pilot informing the controller that he could not maintain VMC. In short, whilst perhaps this particular pilot was always going to do what he did, other pilots might better appreciate that there were alternatives in this situation?
It would be interesting to know the full exchange that took place if that were possible.