Adults aren't responsible for the actions of other adults (generally, and with respect to piracy). So while an IP address can identify a group of adults, it can't ever identify which one was committing piracy.
But corporations are typically responsible for the actions of their employees, that is, they are vicariously liable: http://en.wikipedia.org/wiki/Vicarious_liability. Under at least some circumstances, if a studio's employees are committing piracy, the studio is also liable for that piracy. As long as the IP address corresponds to that corporation, it's "close enough" to signal legal liability (under some circumstances).