This is clearly a popular opinion, but I still fail to see the logic in being obligated to notify your opponent (the shooter) that his shot is being watched by a ref. Notifying the shooter will only give him the advantage of rethinking the shot. Why provide an advantage to your opponent if it's completely unnecessary to do so?
Calling over the referee is to cover your own butt. You're in no way obligated to cover the butt of your opponent.
If it's a matter of sportsmanship...that you want to educate your opponent of the rules...then that's a different story. But in competition, you're in no way obligated to increase your opponent's education of the rules.
With no ref, call goes to the shooter. If I'm shooting and I'm not made aware that the shot will be watched I'm going with my own judgement 100 percent of the time. Although my judgement will be well informed and will be made with the best of intentions. Until a player is made aware that the call is out of their hands the call goes to them. It's not very hard to understand at all.