Why was Oliver released by the magistrate?

Oliver Twist has been put before the magistrate (colloquially known as "the beak") on a charge of pickpocketing. He's entirely innocent of the charge, but, as the boys who were with him outside the bookshop were able to escape, he was left holding the bag, so to speak, and immediately came under suspicion. After a long chase through the streets of London and a punch in the nose for good measure, a dazed Oliver stands before the court, unsure of what on earth's going on. It looks like he's been caught red-handed; the way things are going, he's on his way to prison.
Indeed, the magistrate, the appropriately named Mr. Fang, actually sentences Oliver to three months' hard labor. Thankfully, however, Oliver is saved from this terrible punishment by the arrival of a bookseller. It was outside his shop that the theft took place, and the bookseller is able to state categorically that it wasn't Oliver who picked the gentleman's pocket. As there is no evidence of Oliver's guilt, the magistrate has no choice but to let him go.