And people wonder why they burned witches. Seriously, take a look at the 1st level spell list, and tell me you are comfortable with that. Charm Person? Disguise Self? Ray of Enfeeblement? Unseen Servant? And that's just the first level spells.
In my campaign world, using polymorph on an innocent carries the same penalties as kidnapping and unlawful imprisonment. The penalty for a crime like this varies by region, but includes such things as public imprisonment for twice the duration of the enchantment, being turned into something unpleasant for an equal duration, being sold into slavery and the proceeds being given to the injured party, to having your eyes burned out with hot coals, your tongue removed with an pincher, and your fingers smashed with mallets.
And that's in the civilized parts of the world.
But there is of course a couple complications. Anyone capable of turning someone into something else is a Wizard, and Wizards are in practice (if not in theory) and in some places in fact above the law. (Or at least they are every where that they aren't burned at a stake on sight.) A Wizard that pleads 'self defence' is almost certainly going to be excused, especially if the person in question is of no station or title, the enchantment can be removed, and the Wizard agrees to be moving along without further incident. No body wants to upset a Wizard.
And really, most people would rather have Wizards going around turning people into something unnatural than lobbing fireballs at people. By comparison, offending a wizard and spending some time as a rabbit is alot better than being burned to a crisp or slowly torn asunder by something from the lower planes.
But yet again, there is a complication. If anything happens to a person while they are spending time as a garden slug, its considered murder. This is a more serious charge and it might require some actual evidence that the Wizard was acting in self-defense, especially if the locals can get the upper hand by say binding and gagging said Wizard in his sleep.
Polymorph doesn't carry near the penalties of charm person. Using a charm person on an innocent almost universally carries a death sentence throughout the realm. Even a Wizard of demonstratable power is unlikely to be forgiven for trying to steal someone's free will, and will be lucky not to be murdered in his sleep with the blessing of the local authorities.