The only thing that was ass backwards is the way you went about handling yourself as you left the establishment.
The fact is, your boss told you not to eat tonight, and yet you snagged the first scraps of a meal that was tossed your way.
I, personally, don't care if you licked dog shit off a customers boot. If your boss tells you not to eat...you don't eat. At the very least, you don't act shocked that you are disciplined for disobeying him.
As far as the grapefruit is concerned: always err on the side of caution. Let your boss know that the other manager put the grapefruit up for grabs and see what he has to say about it.
Well...I'm sure apologizing, saying you were a little frustrated over the measures taken for what you perceived to be a minor infraction and admitting that your comment was completely out of line was out of the question, correct?