Seems like you applied the principle correctly here. When you first saw the fence, you did your best to find a reason for its presence. You only concluded that

Seems like you applied the principle correctly here. When you first saw the fence, you did your best to find a reason for its presence. You only concluded that the fence had no purpose after you ruled out sensible alternatives. The principle is not that all fences necessarily have a purpose for their presence, it's that we should assume by default that any fence with an unknown purpose has or had a legitimate purpose at some point, until we are confident otherwise.