I personally think there are distinctions between what you have a right to do and what you should do, and then distinctions between what you ethically should do and what it would just be nice for you to do, and so on.
To me, staying on a WL just to see if you can add another offer to your trophy case, meaning when you have no intention of actually considering it, is not nice. You certainly have the right to do it, and I would hesitate to say it is necessarily unethical. But I think out of respect for the college which is trying to actually complete its class, and out of respect for the other applicants who would actually take such an offer if timely received, it would at least be nice to step out of the way and let them find each other as quickly and efficiently as possible.
By the way, I do think it could be minorly unethical depending on exactly what consequences might happen. That is hard to predict but some colleges do reach the point where they actually under-enroll because they don’t have enough applicants left willing to accept a waitlist offer by the time they make the offer. And in those cases there might have been a match to be made that was never made because of the timing.
This is all very subtle and contingent which is why I would not go so far as to say this is blatantly unethical behavior. But I think it falls into that common sort of area where being nice at least might also have better consequences too, which is another good reason to be nice when possible.