"It could be said that the GPL came into existence to
prevent strip mining of open source, which can be interpreted as
the appropriation of free software code for proprietary gain with
no intention of feeding code changes back to the community. The
part of the GPL licensing framework that achieves this is known to
its friends as copyleft, and is represented by its enemies as "the
viral nature of the GPL", and has been a major factor in the
success of free software."