You see, that's the issue. They want the ad revenue, as that's how they make a huge amount of their money. From a legal standpoint pasting an article so "The Mirror don't get their ad revenue" is entirely unacceptable.
Of course their approach to what is, in the grand scheme of things, a pitifully small amount of lost revenue, is draconian (copyright law is so outdated as it doesn't come close to handling modern methods of sharing data), heavy handed and always likely to be antagonistic. There's nothing forcing anyone to click on the Mirror website and give them money, and actions like this one aren't going to encourage people to do so, in fact they'll result in exactly the opposite.