> Non-pressurized packages (like bottled water) normally contain
  > smooth, non-vented threads since there is no internal
  > pressure to be relieved from the inside of the package.

Actually, some bottled water bottles do have the vents. Maybe to reduce the quantity of plastic in the bottle by another 1% without sacrificing rigidity in more critical areas? In some industries a 1% optimization of part of a process could be non-trivial, especially in a bottleneck component or material.