Since it first became clear that Russian agents spent thousands of dollars a month on political advertising on social media in the runup to the 2016 presidential election, Americans have been asking how the powerful advertising infrastructure run by Google and Facebook could have been thrown open to foreign agents.
But fewer have stopped to ask whether there is a good reason for this infrastructure to exist at all. Why, exactly, is it a good thing for Facebook and Google to be selling advertising to anyone, let alone Russian agents?
The obvious answer

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