The subtle reason is that in a FIS you are responsible for you own seperation In RAS the controller takes on that responsibility. Failure to follow his instructions would mean that he would be unable to be responsible for you.
Again, you need to remember that the controller cannot provide 'instructions' under a RAS, merely information and the advisory avoiding action necessary to maintain separation from other aircraft.
RIS is often a lot more appropriate for brief IMC excursions in Class G - provided that you have the necessary level of mental skill, you can then make your own decisions on avoidance action based on the controller's information, whereas under a RAS the controller will try to ensure a 5nm/3000ft separation against other traffic and provide advice accordingly.
Those who rely upon RAS under IFR in Class G even when in VMC, particularly airline pilots flying to aerodromes outside CAS, might find themselves having to work quite hard if the controller is unable to provide more than a RIS due to traffic density. Although it is possible, of course, that the controller may ask other traffic to fly as requested for co-ordination purposes.