There are 2 different kind of loose stones that you can shovel out, one is the normal loose stone that are the entrance to a hunting zone (eg> trolls east of thais) and others are mission only loose stones (eg, east of the plains of Zao, near the boss teleport) the idea behind it was to protect the quest area so that only the quest doer would be able to get in, while sealing the entrance to people who already did it (eg missions that can only be done once). Since the idea was very well implemented they tried to (supposedly) do it on all loose stone, nevertheless there are some that still works in the old way.