Contextualize not extemporize. If as a member of the military you decide to engage in open political activity in uniform don't go looking for sympathy when the results hit you. In the Army we call it "doing whatever your rank will allow".
A master sergeant is pretty senior he knew what he was doing.
And Grisham's military career is still entirely unrelated to the context of this arrest and its legality, is it not? And I've read this thread, and don't remember much boo-hooing about the status of Grisham's military career. I think most of us understand that military personnel enjoy a much smaller shadow of 1st Amendment protection than the rest of us.