As I said, and that all I have seen posting about this has said, they do normally NOT let it charge to 100%. If they need lets say 60% to reach the next charger, they set it to 100% and return to the car when they expect it to have gotten to 60%. But as you your self have pointed out, it is not easy to be back at exact right time. It it only has got 55% then they will have to wait a bit more. If it has reached 65% - or even 70% - no harm done. From my understanding this is a practice they have had for some time and is not based on idle charging. That it helps prevent idle charing is a bonus. This is also the policy I think I would use myself when I get the Model 3. Even if they drop the idle charing.
If more people will start with this now based on idle charging I'm fine with that. The only difference for they that is waiting is that the charger is actually been used to charge the hole time, and not be idle after reaching a set point.
But I could have agreed with you if they just set it to 100% *and let it charge till it stops* when they don't really need it. But if they do, then the point that it is to avoid the idle charge is moot, as they get the same 5 minutes after ended charging that anyone else.