I think it's a feature, not a bug. On most maps the "castles" are about as impenetrable as thinly sliced swiss cheese, as there are ladders all along the walls, ladders from wall to building, building to building and building to flag, until there is a straight line from the attackers spawn to the flag.
The ability to lay "defensive ladders" puts at least a little bit of sanity back into siege.
In every case I can think of, all a defensive ladder really does is make it slightly harder for the enemy to ladder their way in. e.g., a sloppy ladder that goes over a wall will get rejected, but a well placed ladder that leans against the wall, just below the opening, is impossible to stop with a counter-ladder.
If map ladders are being blocked then that's a problem, but I've not actually seen that.
