This means that while it's possible to sue millions, it's impossible for millions to defend against those lawsuits. The RIAA has practically invented this; a friend in the know confided about 18 months back that the RIAA's litigation was actually turning a profit: that is, the RIAA's network of sleazy bounty-hunters, boiler-room intimidators, and software-generated legal threats were costing less to run than they were bringing in through the persecution of American music fans.
That means that there's no technical reason the music industry can't individually sue all 70 million American file-sharers. Indeed, that might be their last profitable business-model in an era when they refuse to give sell what people will buy: DRM-free music at a reasonable price.
The RIAA strategy is an example of a new legal phenomenon that I have dubbed "spamigation" -- bulk litigation that's only become practical due to the economies of scale of the computer era. We see spamigation when a firm uses automation to send out thousands of cease and disist letters threatening legal action. We saw it when DirecTV took the customer database for a vendor of smartcard programmers and bulk-litigated almost everybody in it...Link (via Michael Geist)
The RIAA uses systems to gather lists of alleged infringers, and bulk-sues them. It has set a price that seems to be profitable for it, while being low enough that it is not profitable for the accused to mount a defence, as they do not get the economies of scale involved.