There's a distinction to be drawn between copyrighted content and copyrighted content of such quality that serves as a market substitute for the legitimate product.
In either case, the DoJ has huge discretion about what cases it brings. A major component of that discretion is its belief that it can win the case (much harder against Youtube); another component is "what offender is so weak that we're likely to maximize the precedential value of the case"; another is "how much of a message will this send".
In either case, the DoJ has huge discretion about what cases it brings. A major component of that discretion is its belief that it can win the case (much harder against Youtube); another component is "what offender is so weak that we're likely to maximize the precedential value of the case"; another is "how much of a message will this send".