The case is now online, and it is fairly readable for non-lawyers, I think. Here's the thrust of the unanimous opinion:
The question is under what circumstances the distributor of a product capable of both lawful and unlawful use is liable for acts of copyright infringement by third parties using the product. We hold that one who distributes a device with the object of promoting its use to infringe copyright, as shown by clear expression or other affirmative steps taken to foster infringement, is liable for the resulting acts of infringement by third parties. . . .
We adopt [the inducement rule] here, holding that one who distributes a device with the object of promoting its use to infringe copyright, as shown by clear expression or other affirmative steps taken to foster infringement, is liable for the resulting acts of infringement by third parties. We are, of course, mindful of the need to keep from trenching on regular commerce or discouraging the development of technologies with lawful and unlawful potential. Accordingly, just as Sony did not find intentional inducement despite the knowledge of the VCR manufacturer that its device could be used to infringe, mere knowledge of infringing potential or of actual infringing uses would not be enough here to subject a distributor to liability. Nor would ordinary acts incident to product distribution, such as offering customers technical support or product updates, support liability in themselves. The inducement rule, instead, premises liability on purposeful, culpable expression and conduct, and thus does nothing to compromise legitimate commerce or discourage innovation having a lawful promise.
And how does one determine such intent? Here's some of what the Court saw:
The classic instance of inducement is by advertisement or solicitation that broadcasts a message designed to stimulate others to commit violations. MGM claims that such a message is shown here. It is undisputed that StreamCast beamed onto the computer screens of users of Napster-compatible programs ads urging the adoption of its OpenNap program, which was designed, as its name implied, to invite the custom of patrons of Napster, then under attack in the courts for facilitating massive infringement. . . . Grokster distributed an electronic newsletter containing links to articles promoting its software's ability to access popular copyrighted music. And anyone whose Napster or free file-sharing searches turned up a link to Grokster would have understood Grokster to be offering the same file-sharing ability as Napster, and to the same people who probably used Napster for infringing downloads; that would also have been the understanding of anyone offered Grokster's suggestively named Swaptor software, its version of OpenNap. . . .
In StreamCast's case, of course, the evidence just described was supplemented by other unequivocal indications of unlawful purpose in the internal communications and advertising designs aimed at Napster users ("When the lights went off at Napster ... where did the users go?" . . .
Three features of this evidence of intent are particularly notable. First, each company showed itself to be aiming to satisfy a known source of demand for copyright infringement, the market comprising former Napster users. StreamCast's internal documents made constant reference to Napster, it initially distributed its Morpheus software through an OpenNap program compatible with Napster, it advertised its OpenNap program to Napster users, and its Morpheus software functions as Napster did except that it could be used to distribute more kinds of files, including copyrighted movies and software programs. Grokster's name is apparently derived from Napster, it too initially offered an OpenNap program, its software's function is likewise comparable to Napster's, and it attempted to divert queries for Napster onto its own Web site. Grokster and StreamCast's efforts to supply services to former Napster users, deprived of a mechanism to copy and distribute what were overwhelmingly infringing files, indicate a principal, if not exclusive, intent on the part of each to bring about infringement.
Second, this evidence of unlawful objective is given added significance by MGM's showing that neither company attempted to develop filtering tools or other mechanisms to diminish the infringing activity using their software. . . .
Third, there is a further complement to the direct evidence of unlawful objective. It is useful to recall that StreamCast and Grokster make money by selling advertising space, by directing ads to the screens of computers employing their software. As the record shows, the more the software is used, the more ads are sent out and the greater the advertising revenue becomes. Since the extent of the software's use determines the gain to the distributors, the commercial sense of their enterprise turns on high-volume use, which the record shows is infringing.. . .
The unlawful objective is unmistakable.
I'll be honest -- this isn't my main area of the law. But I'm happy to discuss. For what it's worth, I'm married to someone who creates intellectual property for a living, so I certainly understand the need to protect her rights to distribute as she sees fit (and protect from infringement).