Mandatory PSN wasn't in the ToS. People bought the game thinking that PSN won't be mandatory.
And if you like to think of yourself as a human being, rather than a consumer built for buying and caving in to ridiculous corporate demands, you shouldn't need any other reason than simply stating "I don't want to, fu*k yourself".
Also, even if you don't care, PSN isn't available in most of the world. It's only available in 69 countries. This means that those players who bought the game can no longer access it. Sony even delisted the game in those regions.
And in case you don't recall, Sony has a pretty bad track record of incompetence when it comes to keeping their customer data secure. PSN was hacked twice. Do you really want to give your data over to a company that can't keep it secure, even if the game that you're trying to access can run perfectly fine without your data?
And as a cherry on top, PSN is still optional in China and Russia, which proves once again that it isn't necessary and that Sony execs are basically giving you the middle finger and telling you that you'll suck it for as long and as deep as they want.
If that's who you are as a person, more power to you. Go swallow that metaphorical corporate load.