Well, that would be an example of another change that could be made. That water should be able to be contained and pumped overboard unprocessed.
The intent of the law (no more giant oil slicks and environmental damage) should be the end goal. 15 PPM makes that a lot harder to accomplish. So, the equipment gets bypassed and more damage results than if the limit was just set at a level that makes the engineering easier.
Volume wise, most oil and gas boats could keep under a reasonable limit especially with good engine room design.