Free as in bombs
by David "Novalis" Turner
How are GPL libraries "free", I am sometimes asked, if we don't permit proprietary software to link to them?
After all, Microsoft permits any software to link to their libraries (in most cases; some recent Microsoft libraries don't permit linking to copyleft software). I just got email suggesting that if someone linked proprietary software to GPL software, FSF would "kick down [his] door and turn over [his] desk." This isn't quite accurate; usually, I prefer to use grenades to open stubborn doors.
More seriously, we don't choose to use the criminal provisions of copyright law to have GPL violators hauled off to prison. We don't even usually need to call our lawyers. But Microsoft has had people imprisoned for violating their copyrights. My job would be a lot more fun if I were a commando. I would cut my hair short and smoke cigarettes and carry only small knives to ward off insinuations of compensation.
But back to the question: why are Microsoft's licenses occasionally more permissive? Because their goal is to get everyone using these libraries, which only work on their operating systems. It's just like a drug dealer offering the first hit for free. By contrast, FSF's goal is to have all published software be free software. We choose our license terms to advance that goal. You could also say that bombs are free to their targets, because the targets don't need to pay for them.
So, the answer is that we describe a license as free if it permits certain actions under reasonable terms: basically, copying, modification, and redistribution. We've even got a definition of free software. If you want to know whether we consider a particular set of terms reasonable, check the license list.