Most GPL projects aim for a diverse copyright holder base, with the code solely licensed under the GPL, so that no one entity can escape their GPL obligations by paying money. In that scenario it becomes harder to enforce the GPL, unless you have a principled copyright holder base with deep pockets. Hopefully the Software Freedom Conservancy lawsuit mentioned upthread will change that, they aim to set the precedent that anyone who receives GPL violating binaries can file GPL violation lawsuits.