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?)
Indeed.
"Your honor, the defendant is distributing my copyrighted work without a license. Make him stop please."
Most lawyers who read the GPL are not excited to go to court. They will advise you are settle without going to court. Lawyers generally want to feel like they have a chance at winning a case in court before bothering, it doesn't feel good to lose - even when you know in advance the case is lost.
Don't read that as lawyers never go to court unless they will win. This is obviously not true: there are many reasons to take a case to court knowing you will lose it. However if a case will be lost in court they will avoid court if possible.
The OpenWRT project is the result of a GPL enforcement action in the USA against Linksys and Software Freedom Conservancy is planning another enforcement action in the IoT space:
https://sfconservancy.org/copyleft-compliance/enforcement-st... https://sfconservancy.org/copyleft-compliance/firmware-liber...
My non-expert impression has been that open source licenses and "licenses" that aren't ... explicit contractual agreements between parties (i.e. not just click through or some text in a download) do not have a particularly strong backing with law or case law. Whether or not you "can or can't" do something with regards to effectively making up rules that will be enforced by courts with a blob of text in source code or a check boxes on websites is still somewhat up in the air (somebody who knows things feel free to correct my impressions).