Few, if any, of the defendants make their own mapping software. Move.com, for instance, uses maps provided by Microsoft’s Bing. But patent laws allow a patent-holder to sue anyone who makes, sells, or uses a patented item. In this case, it’s the real estate companies, together with their users, who are alleged to be “jointly infringing” the patent.
via Gone house hunting online? Revived patent lawsuit says you’re a “joint infringer” | Ars Technica.