But then Wisconsin would be obligating every website in the world to block every VPN in the world.
No, just adult ones.
I'm pretty sure you can see how that's not legally enforceable.
Like I said, some sites will go to this level (as in they already do) and some won't. It just depends how popular and visible the site is and how likely they feel they are to be targeted by an over-zealous attorney general, and if the potential fine is larger than the lost revenue or not. And it doesn't even have to mean revenue loss. They could just say "you're connecting from a VPN so you have to provide your ID regardless" so that they're compliant.