It's true that the GPL, for example, rests on copyright law. But it is also true that the GPL is about sharing, so someone in the FOSS crowd may well be of the seemingly contradictionary opinion that its the right thing to do to violate copyright when it comes to sharing culture to enrich the lives of people who can not afford everything, but that it is the wrong thing to do to use GPL:ed code without sharing the modifications source.

Edit: I don't pirate software. Software piracy hurts FOSS. If it weren't for SW piracy, more people would use FOSS. Regarding music and such, on the other hand, I might be persuaded to believe that legal and moral aren't necessarily always the same thing...