I think the commenter above was trying to say that the "you have to enforce it every time or future cases are weaker" applies to protecting trademark rights and not copyrights. (They weren't saying this is a trademark case, they were saying that because this isn't a trademark case, trademark rules don't apply.)

The "you have to enforce it" thing is about genericized trademarks: I can sell a painkiller under the name "aspirin" because everyone forgot it's a trademark and so there's no argument that there's an intent to cause market confusion with Bayer. But the copious pirated copies of Windows don't let me sell it without licensing it from Microsoft.