Your answer is interesting and worth it. (+1) :)

Adblocker: a resource that blocks, for you, ads in the internet. That modifies the way you view the content someone publishes.

Piracy (in informatics): illegal use of copyrighted content, and, more specifically, using it for personal profit, like selling copies.

As long as sites don’t (usually) come with a contract you have to sign, and no law forbids adblockers, it is a legal option to visit online content. Like, you can use lynx (text-based browser), so you don’t see webpages as graphically planned, and blind people only listen to the content, in both cases not seeing ads, or not all of them. Would we call it illegal for some reason? If not, why not showing ads would be different? (I imagine someone cutting ads on a magazine to give another example of you’re not obliged to visualize or even read what you don’t want to.)