I whole heartily agree. Those that don't are a very rare exception. The older I get and the more interaction I have with people the more I affirm Trust No One. <—That's only very slightly sarcastic.PEOPLE SUCK!
I don't have a top box or I'd use it all the time. Carrying my helmet doesn't appeal to me though it's second nature after all these years. Other's anecdotes of how gold ingots left on the bike are unmolested don't help me if my helmet is stolen. This happened to me once on base when I was in the Mil. I learned then and try not to forget.
Here's hoping you get your hat back. I'm all for pushing for prosecution even though a local DA may say it's not worth their time. His/her call of course. The DA may proceed without your prodding. Being the petty guy I am I'd want to at least inconvenience the thief as he did me for his moment of stupidity. To do nothing just gives him tacit approval.
Best of luck to you..