endplay
You have asked a valid question, and it deserves a reply.
In the Chinook case, my own opinion is that it is possible that the pilots were "to blame", at least in some measure, but that the evidence available does not prove that that is so, even beyond a reasonable doubt. Much of it is mere speculation, or at least "informaed" speculation based on modelling, after the event, of parameters that cannot be established as factually accurate. Since the pilots are not here to give their version of events, and none was recorded at the time, we should not hold them to have been negligent. They were responsible for their own terrain separation, but why they did not maintain it we are simply not able to say.
In the F-15 case, we likewise do not know for sure why the aircraft crashed. We do not know for sure whether they were in cloud when the en-route clearance was requested, or if they intended to convey this to the controller. The RAF Board and the USAF Board use different interpretations of the voice recording. Even if they were, we do not know for sure whether the pilots achieved VMC before impact or not. Once again, they were responsible for their own terrain clearance, and failed to maintain it for whatever reason. They should not be held negligent, but on the same evidence neither is there any case for the controller to answer.
Regards
Ginseng