The White House has communicated in the past that these *are* official White House communications... so when Trump starts spouting out this "Kill Hillary!!" stuff, that's the Commander in Chief of the US Military communicating "lawful" orders to every member of the US Military (among others).
If push came to shove, a case could be made for members of the military refusing his orders in a time of war. oh... what's the penalty for that again ?