Yeah, I'd have to say that the only way to get that to be walkable is by using triggers. The reason that it isn't walkable is because a Line of Sight blocker blocks what the player can see in that area. If your unit moved over it, the unit's acquisition range would drop to 0.00 and his Sight Radius would also. So pretty much, going on a Line of Sight blocker is totally pointless. That is why Blizzard figured it'd be a good idea to make it unwalkable. No idea why it's still unwalkable after choosing the walkable option. Maybe changing the pathing texture or pathing value to "NONE" will make it walkable. If not, then triggers are your only resort.
~Craka_J