You can't force the user not to modify the data he fetches, as long as it's for "his/her personal use only". That's the negative side of things.
All in all, I would put it this way: if a user thinks he/she wants to use an addon that displays third-party advertisements instead of the advertisements that originally were displayed on the website, it's their way of opting-out of your website's advertising and opting-in to other advertising at the same time.
I personally can't see the value of this, but they apparently do see some value. I agree that it might be frustrating, depending on the point of view that something like that exists... but it's nothing else than sticking a post-it on your monitor screen with a big smiley, where actually some advertising area would be displayed. Is it logic? No. Do people do it? Yes, I wouldn't have imagined that myself either - until I saw it happen the first time and hit myself on the forehead while asking myself about the reasoning behind that.
I guess it's in the nature of things. Like Murphy's law: if they can mess with things, they will. But they also have every right to. If not, running a website which displays "forced" advertising could be compared to a little "dictatorship"... and (hope you've been following the news the last few months) we've all seen what happens to dictators as time goes by. ;)
As for the legal side of it all: too many countries with too many potential visitors being able to access and browse your website makes "the law" a hard thing to discuss. After all, even US jurisdiction would run into trouble taking a side on questions like these. In the end, it shows one of the qualia of the internet: it can't simply be pinned down, not even a little.