GNU GPL - Copyright helps Copyleft
Problems of GPL Enforcement

Problems
distributed copyright
is an important safeguard
can make enforcement difficult, since copyright traditionally doesn't know cases with thousands of copyright holders
distribution of damages extremely difficult
the legal issue of having to do reverse engineering in order to prove copyright infringement(!)
only the copyright holder (in most cases the author) can do it
users discovering GPL'd software need to communicate those issues to all entitled parties (copyright holders)
infringers obfuscating and/or encrypting fres software as disguise