A: Because he stole home!!
Explanation: The humour within this joke lies in the double meaning of "stealing home". In a baseball sense, stealing home is a completely legal activity, encouraged by the powers-that-be. However, outside of the baseball world, stealing home is illegal, and an infringement on a citizen's right to private property. However, the policeman in this joke fails to see the difference between the situations, so he arrests Hatcher because he's been brought up in a non-baseball world, where stealing home is illegal. Although it isn't directly stated, it is implied that eventually the misunderstanding was cleared up, and that Hatcher was released from custody.
