Yankee_Doodle_Floppy_Disk
It is very simple, if you are on an IFR flight plan ONLY the pilot can request to go VFR it can not be imposed on him by ATC.
The only person on earth that can prevent the aircraft from flying into terrain is the pilot. Regulations recognize this and have put the final responsibility with the pilot and not ATC. Crews have the maps with all terrain info in front of their nose. ATC has to monitor but do not have the final responsibility.
Since ATC have a complete picture of all aircraft and not the pilot, ATC is responsible for separation.
It is that simple.
In practical terms both pilots and ATC monitor each other as much as possible but when an incident gets into the courts the rules apply.
Some ATC chaps pick you up and state "radar vectoring; intention visual approach" and that's fine with me since I will be prepared or I can refuse at an early stage.