They quote the same consultant who was willing to go on record making that same implication for the New York Times. Consultants like publicity, so here's how it's going to work with him: As long as he can get a quote published in a newspaper for that opinion, he will continue to have it. And if you're a reporter who doesn't want to bother actually doing any work, you'll call him.
If you're a reporter who wants to keep in tight with your incredibly anti-Amazon bosses, you'd be sure to give this speculation the puppet-master spin. Because when someone does something illegal in an attempt to harm someone else, and their victim calls the police? That's eeeeeeevil! Almost as bad as donating to charity!
What would happen if you were a reporter who actually wanted to do a little work? Not, like, actual reporting--clearly, that's too hard--but maybe just a quick search of existing articles written by others?
Well, you might discover that story published in 2010 in the New Yorker that was all about how publishers were colluding with Apple to fix prices. You might discover Steve Jobs bragging about how he got publishers to collude to fix prices and openly discussing what their prices will be. You might realize that Amazon's dispute with Macmillan was hardly conducted in secret.
And you might, like Passive Guy did, discover this article in the Connecticut Law Review, which outlines the process behind the Connecticut attorney general's decision to investigate agency pricing. (Oh, you didn't know it wasn't just the DOJ? Sixteen states are also involved. Clearly, Amazon's eeeeeevil! reaches everywhere!)
How did they decide to target agency pricing? Did Bezos call them in the middle of the night? Has he planted chips in their brains to submit them to his will? Are we all in the Illuminati together? (Well, yeah, of course we are, but that's not important right now.)
Oh, it turns out that the head of the antitrust division reads the New Yorker--he gets some of his best leads from newspapers and magazines, because people who break the law often have an odd compulsion to brag about it. According to the CLR, the article "practically drew a treasure map for the antitrust investigators." It also quotes the head of the antitrust division as saying, “This one wasn’t something exotic like an inside source.”