Honestly, all you can really do is post a link to the manual that says that they're active duty. They can either accept it, or not. And if it's a not a government organization, there's not much you can do in the end.
You essentially get one "but sir..." here. It's an easy mistake.... it only applies to cadets and midshipmen at USCGA, USMA, USNA and USAFA (so not USMMA or ROTC). And you've got to decide how "far" you want to go with this. As soon as you post "What about CGA cadets" you will often get two ignorant responses..... first.... the Coast Guard isn't the military, and second, cadets aren't active duty. Both are wrong and both responses will make you want to respond.
And this won't be the last time. Each year you will dread trying to take advantage of any offers, such as the free Veterans Day lunches.... because at some point you'll get a "Coast Guard isn't military" reply and your blood will boil. It happens. Ha!
Quote the policy, maybe they'll agree, maybe they won't. If they don't.... buy somewhere else, but don't make a campaign out of it. I'm not sure how much a cadet or midshipman really needs an Oakley SI account either way.