Conversely, while the police are a violent and nasty brute squad, if they come upon the thieves already restrained and disarmed, incapable of putting up a fight, they are probably not going to do anything that permanent to the thieves besides "arrest them," which they were going to do anyway, especially if they're smart enough to follow commonsense advice. The police do all kinds of heinous shit to thousands of people every year, but they arrest millions of people every year, so while the odds are a hell of a lot worse than they ought to be, this isn't a deterministic case of "if we abandon them tied up they'll totally get shot in the head" or something.
Plus, if they're taking civilian hostages, they're already about as morally bad as, like, Worst Police. This is some charcoal grey on charcoal grey morality we've got here.