GPL / GNU

I’ve seen a lot of custom blog theme shops that have Copyright notices like the one below, when the themes they are selling are child themes of TwentyTwelve.

That would mean that they are protected under GPL / GNU correct? Do these copyright claims have any validity to them? How does someone who is selling themes go about copyrighting their work if they are simply modified child themes?

“our themes are copyrighted blogzilla, prior to being sold as well as once purchased. you are strictly prohibited from removing or altering the copyright notices on the bottom of each theme. you are prohibited from claiming the work as your own. you are prohibited from sharing the theme or any design elements we provide with additional users. “

Many thanks!