Copyright infringement is a strict liability tort in the US. Willful infringement can result in harsher penalties, but being mistaken about the copyright status is not a valid defense.
I don't know if you're trying to say that, in the realm of tort law, it is only strict liability, or if you are saying that copyright infringement is only a tort. If it's the latter, it's completely untrue, as there are criminal copyright infringement statutes.