At the risk of sounding difficult, all work is derivative and I think that's a beautiful thing, not a thing meant to be haggled over and fussed about.
But I do get the point of your second paragraph. Ideally students go to class for the sake of learning, so it doesn't make sense to blatantly cheat, but such is not often the case IRL.
Also posting the solution online would obviously make it difficult to use for the next year and I don't think it is productive to make a professor remake an assignment every year when the could be focusing on refining lecture content.