The police know the person is a thief, but they can't get a warrant from the court. Firstly, the thief hasn't acted yet, so a warrant can't be issued. Secondly, the police might not have enough evidence, even though they know there's been wrongdoing but the evidence is weak. Thirdly, the thief might not leave any evidence, even after committing a robbery. A thief is a thief. In the past, some political criminals have had more power and influence than the law. They have the power to make laws stricter or lighter, and change prison regulations. In the end, they escape punishment.