[charcircuit]

Yes, they are legally obliged, but they can just tell you to pound sand and you can't do anything because you aren't the copyright holder.

[ColonelPhantom]

Then I contact the copyright holder (or one of the many, in case of e.g. the Linux kernel). They probably care, or else they would not use the GPL. I also believe there are organizations that can help such as the Software Freedom Conservancy.