Or maybe because you have to. There was a case of a free software
project (JMRI) being sued for patent infringement by a proprietary
software company. It turned out that the proprietary software included
source code from the free software project without attribution
(copyleft was not even necessary, as the project was under the
Artistic License!). In this case, the possibility to counter-sue saved
the free software programmer from having to pay millions of dollars.
I think this might be an over simplification. There were many statements
in this history (new to me - just read it all - good read) that
demonstrate that the patents were incorrectly granted. The copyright
issue was involved, and the defense of free / open source copyrights was
involved, but it looks pretty clear to me that JMRI wanted to shut down
*all* violations. They wanted the incorrectly granted patents dropped,
and they wanted their copyrights held intact. Was the latter required
for the former victory, or was that just how things played out?