"Not". You keep using that word. I do not think it means what you think it means.
I really am not sure what you mean. 100% of all GPL code means you can modify it to your heart's content. Any code you get from wordpress.org is GPL'ed and that goes with the philosophy here too.
The GPL covers the use and distribution of the software. Meaning you can edit it, use it, do what you like with it. No restriction there and all of the themes at the repo are GPL'ed.
It also means that if you distribute it you can't restrict your users from doing the same thing. So if you take one of these themes, modify it and let other people use it, you cannot prevent them for enjoying the same freedom you enjoyed.
If you do not want to use one of those themes that's fine. But please don't represent that those themes are restricted. That's simply not the case.
Also regarding other free themes located via search engines: there are many good and decent people who do provide their themes free like that themselves.
BUT they are outnumbered by about *quick calculation, sips more coffee* 100,000 to 1. The other 99,999 insert really dodgy code into the theme. You basically hand over your WordPress install to spammers and it's a big reason why we never recommend anyone get themes from elsewhere.