Of course it is legal. Is it illegal to throw away the advertisements from your newspaper before you read it?
That is a more accurate analogy. It's stealing and depriving the publisher of their income.
In any event, getting past adblockers is really really simple. It always will be, and if you think an adblocker can block anywhere near all adverts, you should go read up on how things work.
To suggest that blocking an advert is stealing assumes that the author of the web page would make money off that advert in the first place. Furthermore, that logic suggests that anyone who chooses to ignore advertising rather than just blocking it is also guilty because they're depriving the publisher of a potential income too. Total rubbish.
You are stealing bandwidth, processing power, storage, electricity, etc
Society is about give and take. Adblocking is about take.
Do you click every advert you see just to make sure you're compensating for 'stolen' bandwidth, processing power, storage, electricity?
FYI: I redistribute scripts under GPL licensing. I don't ask for donations, nor do I litter my pages with advertising. I consider that pretty "giving" - and no, I don't have problems with people 'stealing' my bandwidth, processing power, storage or electricity.
By blocking all advertising on all websites you're sticking your fingers in your ears.
Personally, I find adverts useful. Websites that resort to crappy user experience adverts - popups/unders/flash etc usually aren't worth visiting in the first place.