Theft is theft. It's that simple. Registering it with the US Copyright office can later assist with such documentation, but that alone provides no other protection. Nobody, anywhere, will attempt to protect your work from theft. The purpose for and circumstances under which you used the other guy's property are of little importance.
Theft is theft.
You may well be expected to pay financial damages far in excess of anything you might consider reasonable. Attorneys specializing in prosecuting copyright violation are extremely aggressive, they make big bucks, and if you lose the case, you'll pay their fees. DO NOT expect your ISP to defend you. It just won't happen. Goes with the territory.