Bank robbery is not a petty crime. In fact, it's a felony.

Even so, the judge is sure to look at the circumstances and say that he was clearly doing this to be institutionalized. I think the usual response in those cases is to say "you did this to be arrested and incarcerated, so I'm not inclined to reward that behavior by giving you what you want; but if I do not, I have every reason to believe you will commit a far more serious and earnest crime."