[joeysteele]

Because once someone lashes out,it can be uncertain as to how far their anger takes them.
Yes he only hit empty pizza boxes,however with so much aggression behind the 'physical' force used on those boxes, it could be unclear what may have happened, had say someone tried to restrain him.
He then could have lashed out at them too since he was heading out of control.

I have seen,rarely thankfully, people argue in a pub, one of them swipe the glasses off the table and then punch someone who then tried to restrain and reason with them.
It was obvious to me why he got the warning, keep things verbal not physical.
That is all, and what he should have done but failed to do.