The Internet was built on free content. It was only later, well after first seeing the marketing/branding potential of having websites, that companies tried to directly monetize what had been provided free for years.
But never mind that, blocking ads isn't theft or even slightly illegal, any more than skipping commercials on a DVR is, or reading substantial portions of a book at the Barnes & Noble. The way the Web works is that the browser has the power to render pages according to however it's programmed, not according to what the site owner wants. If they really didn't want anyone seeing their content without paying for it, they shouldn't be serving it out on the public Web to begin with.
(And there are plenty of people who think it's perfectly morally acceptable to get all sorts of content for free, so long as it only involves copying data and not stealing physical objects — but I'm guessing that's not what you were talking about.)