Those are passive billboards. And very few people would mind if the advertising on the web was just as inobtrusive. Initially, it was and generally we accepted

Those are passive billboards. And very few people would mind if the advertising on the web was just as inobtrusive. Initially, it was and generally we accepted it as normal. But ad brokers began making more aggressive ads, with animation, sound, and pop-ups. Thus the demand for ad blockers. Physical advertising has to obey sign ordinances and in some places laws regulating the content of advertising. (In the US this is loosely enforced and mostly just for food and drugs.) You wouldn't be able to get away with constructing a billboard directly in the right-of-way of a road in an octagonal shape colored red with the word "STOP" on it. But that's the equivalent of what internet advertisers do with their fake virus warnings and the big green "Download" button.