I can't understand this logic.

Assume as stated that a website is a copyrighted and protected. Sure, that means I can't redistribute it to others without permission or a license. But I can't see how me locally, privately modifying the site would be against the law. Should Crayola be sued because their crayons can be used to modify a copyrighted art piece? Is it illegal for me to watch a movie with a blue-light filter on because it modifies how the content is displayed?

Edit: After further thought, a stronger argument would be that it's illegal (in some places) to bypass DRM protections. That's because if I break DRM of some media (say, of a rented DVD) so that I can keep it forever, that would technically be illegal even if I never shared it with anybody else. So if a site tries to break ad blockers but an ad blocker works around that, that would be "breaking" DRM, therefore illegal. But I still find that to be an lacking argument.