They're violating the license, which means they no longer have the right to distribute the GPL'd code. In some cases, they may have to pay damages. However, in

They're violating the license, which means they no longer have the right to distribute the GPL'd code. In some cases, they may have to pay damages. However, in the US (laws may be different where you live), the violator can't be forced to divulge their source code; the author of the GPL'd code has no right to it. An analogy might help; say I distribute CDs, with 6 songs I wrote and 6 songs by the Beatles, without a license. A judge can place an injunction to stop me. The copyright owner might seek damages. The judge might put me in jail/prison, if I continue infringement or don't pay. But they can't force me to give up the copyright on my songs, merely because they're distributed together.