having lived with terriers for the the last forty or so years I've yet to find an answer! I keep telling myself they are just trying to be helpful, and certainly if I dig then they dig as well, problem is they are so darned enthusiastic about it, and not always where I need help.
I have noticed that any particularly well manured areas are greeted with relish, made the mistake one year of trying 'Rooster' pellets, not repeated. Good luck with the thorny barriers, so far nothing seems to discourage mine when in the mood (it does seem to lessen with age, maybe)