The Scorpions

I really don't think that the Scorpions are worth launching a debate about morals vs. aesthetics; from a purely legal standpoint, however, the image is not illegal, considering that (as the article points out) the FBI never came to a conclusion regarding its legality.

The FBI does not determine legality. That is up to the courts. The FBI determines if a case is worth pursuing and if a conviction is likely. The agency obviously decided in the negative on both counts. Until a court rules otherwise the image is legal and that's a central point of the story.