When using Zeus to specify the contents of an ammo box, bi-pods are not in any category to select them (one would expect in accessories or items). There are weapons with bi-pods but not just bi-pod as an individual item. The one ammo box I have found that does contain them by default (supply box), when this box is edited with Zeus, the bi-pod is not in the list of items.
No problems with Arsenal.
No traction on this question in the BI forums. 150+ views and zero response. I don't know if I'm the problem or this is just a current game limitation.