That is strange, because there is no such check in the code. Is something else different about them, or the plot they are on? Or maybe it's just not a PvP game?
Definitely PvP (I've had my hut destroyed a few times by one of the other players (huts seem to break quite quickly now, it certainly didn't take 8 damage to destroy it each time)) and others seem to have been able to attack the fences (on common land) next to my plot, but I only get repair or improve options on any of the fencing (I was wrong first time, I've now looked and it's the same for fences, wood walls and stone walls), no attack option.
