As a former USMC Sgt, I can tell you that there are many, many ways by which his superiors can make their displeasure known without "official punishment".
The decision to "keep it in-house" was probably based more on legal issues, not as an "excusing" of his behavior.
With the state of society in both our democracies, formal punishment would have been followed by a claim for monetary compensation for "physical & emotional damages" from the a$$hole civvie... and would signal an "open season" for those who want a fast buck to try ever-increasing levels of harassment of Guards to try to provoke a reaction that could then be sued over.
No, this is exactly the right way to handle it... put a damper on any cash-hungry a$$holes & solicitors, while making the individual who lost self-control/discipline pay for his indiscretion out of the public gaze... and he will be punished, make no mistake about it!