Copyleft Through Copyright

The primary goal of every GPL enforcement action is to gain compliance, which means getting to users complete and corresponding source code so they can copy, share, modify and install improved versions. The GPL itself is a copyright license that does a weird hack on copyright: it uses the copyright rules to turn them around, and require people to share software freely (as in freedom) in exchange for permission to copy, modify and distribute the software. A GPL violation occurs when someone fails to meet the license requirements and thereby infringes copyright. The copyright rules themselves then are the only remedy to enforce the license — requiring that the violator come into compliance with the license if they want permission to continue distribution.

Up until now, almost all the enforcement I've done has been purely under GPL version 2 (GPLv2). GPLv2§4 says that upon violation, the violator loses permission to engage in those activities governed by copyright: including copying, modifying and distributing the software. The only way to get those permissions back is for the copyright holder to grant them back.
Speaking For the Users

Copyleft's unique way of using copyright means the parties who may enforce are copyright holders (and their designated agents). However, the victims of the violation are typically thousands of users who have bought a product that included the GPL'd program. The goal, therefore, is to get source code that these users can actually use to compile and install the software. In GPLv2-speak, the goal is to get the all the "complete source code", which includes "the scripts used to control compilation and installation of the executable".