Because we're your family, friends, neighbors, local shop owners and other people you share society with? Most AdWords advertisers are people running small businesses. "Advertisers" is not a separate class of entities that doesn't intersect with other people you care about being fair to. It's not morally OK to purposely spend our money as if it were yours just because nobody will stop you.

They are the people, ultimately, who pay for the content on those websites. Although, I totally agree that the advertising model is broken. When a blog's insightful articles are paid for by the sensationalist blogspam, we've got a problem.