It's a classic case of someone's hubris blinding them to reality; they need Google a LOT more than Google needs them.
I somehow doubt that Playboy, Inc, needs Google (or boing boing) providing links to illegal copies of their work being posted on Imagur. I think I it is quite reasonable for Playboy to go after those who have done, or assisted in doing, that, and Playboy would not be harmed in any way by Google (or boing boing) taking down all links to that content.
I assume the legal argument is that boing boing NEW that the material it was pointing their readers to was an illegal copy, and thus boing boing was aiding the copyright infringement by advertising it on the behalf of the person who scanned and posted the images.
It would be like, I think, someone who told other people where to find the bootleggers so they could buy moonshine. I assume that this is a violation of the law, but maybe not. I mean, apparently, actual moonshining and bootlegging isn't since the feds seem incapable of watching the television show "Moonshiners" and identifying and arresting those who go on television moonshining and bootlegging. What better evidence do they need?