The US Supreme Court decision is remarkably stupid, or at the very least extremely inconsistent.
Justice Souter said ""We hold that one who distributes a device with the object of promoting
its use to infringe copyright…is liable for the
resulting acts of infringement by third parties." (As quoted here)
In other words, those who produce file sharing software are responsible for illegal things done with their software.
How about if I rephrase it as "We hold that one who distributes a gun with the object of promoting its use to shoot people is liable for the resulting shooting of people by third parties"?
Does all use of software for malicious purposes then possibly make liable the creators or distributors of that software? What about using Visual Basic to create viruses, or Microsoft Word to write extortion letters?