I suspect there is another reason to consider about reticence to discuss where the line is and what exactly the plan is once crossed: the POR against insurgency is find, fix, and destroy [0] and there has been plenty of domestic [1] practice. Why make yourself an easy target?
So, to your first point that they haven't thought about it meaningfully, I agree. Fashion items.
[0] http://www.globalsecurity.org/military/library/policy/army/f...
[1] http://www.npr.org/sections/codeswitch/2015/05/18/407665820/... etc