Under what conditions are passengers required to take precautionary measures during refueling?
Doc 965, CAT.OP.MPA.195(b), regarding Refuelling/defuelling with passengers embarking, on board or disembarking, says:


  For all other types of fuel, necessary precautions shall be taken and the aircraft shall be properly manned by qualified personnel ready to initiate and direct an evacuation of the aircraft by the most practical and expeditious means
  available.


So no-belts restriction is one of the elements that ensures "expeditious evacuation". The same goes for keeping the doors open and keeping the passengers in their seats. 

Although the legal requirement is not that specific, probably the detailed procedure (with all the elements you described and others that were not visible for the passengers) can be found in the company Operations' Manual.