Ten years on the force taught him that possession was nine-tenths ownership by normal law, but Barry didn't live by normal law.
He lived and worked within the shrouded world of incorporeal law. The slightest percentage of a possession was a violation illegally obtained.
It was his job to retrieve possessed bodies and give them back to their rightful owners while bringing those who illegally possessed them to justice.
He wasn't just a paranormal cop, he was a Demon Hunter.