Publishers, like anybody else, can always hire an assistant to help get things done. That assistant, in legal terms, would be an “agent:” that is, someone acting on behalf of another person, in this case, the publisher. If a publisher hired an agent to help sell books, the publisher could of course, tell the agent at what price to offer the books. (Just the same as if you asked a friend to help sell your car, and you told your friend what you wanted to charge for the car; or you asked a real estate agent to help sell your house, and you told the agent what to charge for the house.)

That’s what Apple is proposing: that it will act as the publishers’ agent. Apple wouldn’t really function as a “retailer” at all–it would simply be acting on behalf of the publishers, the way a real estate agent acts on behalf of home sellers.

There’s anything at all legally wrong with anybody running a business in that way. I think the DoJ’s complaint isn’t (or shouldn’t be) that that MODEL is illegal. It is, rather, that Apple and the publishers colluded in some way that antitrust law doesn’t permit. I don’t see how the DoJ could complain about price fixing in particular, though, because nothing in the agent model of book sales requires any particular price.