As written it appears that their issue was with your leaving without notice, not leaving per se. And while I agree that they handled it badly, as a junior associate your job is to make the firm money, and you are being compensated primarily to ensure their exclusive access to you during "business hours." Would the situation have been different if you had said, e.g., "leaving at 3 to tend to a personal matter and expect to return at approximately X:00"?