Doesn't stop them from saying anything. It just ensures that whatever they say will not, in any way, reflect US government policy.
At the very best, those news agencies proceed to run material from our enemies' information ministries and news services.
At worst: they simply report that the Pentagon refused to comment on its activities, casualties, or anything else. And they say that day in and day out.
The American public would, eventually, start wondering Just what is the Pentagon so damn afraid of? Are they engaged in questionable or unethical activities? Why are they so hell-bent on covering up their own deeds?
Congratulations: you've just succeeded in completely cutting off the Pentagon from the American public, and thus denied them the support of the American public.
You work up some steam, and then with all your might you knock the straw man to the ground. I love watching the Three Stooges when Curly gets excited and starts running in place, slapping his face over and over. Good stuff.