Please note that without any license, the default in the US would be all rights reserved for the copyright holder, and thus does not permit you to redistribute

Please note that without any license, the default in the US would be all rights reserved for the copyright holder, and thus does not permit you to redistribute without author's permission. Therefore, that particular argument alone (without some additional fair use or other claim) would likely not fly in court in the violator's favor (i.e. if you did not agree to the GPL attached to it, which license did you think you have that permitted you to redistribute the copyrighted work?)