Why so they let the pirates take over the stockade?

The doctor explained the to Jim that two months before the Hispaniola appeared, Ben Gun already had the treasure safely stored away in a cave. He had come upon the skeleton, rifled the positions, found and dug up the treasure, and carried it to his cave. After the doctor heard Gunn's story, Livesy allowed Silver to have the treasure map and the stockade - the rest of the party moved to the cave to guard the treasure.