Under the University of Massachusetts at Amherst, if someone else uses your router as a means of copyright infringement, you are also held responsible for that infringement. In fact, they trace the infringement only to the mac address that is registered on their network.
Not 100% sure on why this is, but my best guess is that the state urges UMass as an ISP to prosecute copyright cases. In many cases, infringement notices are handed out to students, who cannot access the internet on campus unless they deal with these infringement notices.
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