For my part - well, I think the reason Stallman can come across as a religious zealot is partly because he opposes something ("Intellectual Property") that is arguably religious zealotry itself. The problem is, the theory of property he opposes has gained the status of a "state religion", to the extent that most of us don't even see it, and when someone points it out to us we think they're a nut. But that very pointing out is probably Stallman's biggest contribution.
Even fellow hackers can miss this point. Some take the view that the best approach is to leave your own code open, if you choose, but still give others the "freedom" to lock theirs down if they want to. As Stephen Turnbull said in discussing the XEmacs/GNU Emacs fork:
"... many XEmacs developers take a different approach to the free software movement itself, and so will differ from GNU/FSF policy. The code we write will remain free. What we are possibly losing is the ability to force others to make their modifications free. This is central to the GNU Project because it is a social movement. To many developers as individuals, it is not so crucial.""Isn't that the most free way?", they'd ask; "Make our own code free, but don't force anyone else to do the same." On the surface, who can criticize anyone for being so magnanimous? But the thing Turnbull's comment may be missing is that the "force" applied by something like the GPL is force applied only against other force. The only thing the GPL is doing is stopping you stopping someone else. The GPL would not even be necessary were it not for the fact that current legal structures allow you to restrict other people's freedom. The GPL is not really an enforcement; it is a de-enforcement.
So it all boils down to this idea of property, as applied to code. IF we accept that a person can "own" code, then clearly Stallman is wrong. Making "your" code freely available may be a laudable choice, but it's a choice. It is, after all your code. But if Stallman is right to argue that code doesn't belong to a person - even the author - then any attempt to use force (and let's face it, all laws are eventually backed by threat of force) to stop people using it is plain old immoral.
Stallman's position appears to be (but hey, what do I know, I'm not Stallman) that putting a gun (or a having a cop do it for you) against someone's head for using code that you wrote is the same as putting a gun against someone's head for using the word "Stallman". Putting a gun against someone's head for any such reason is wrong. Immoral. And why? Because it denies people their freedom (as in, "we hold these truths to be self-evident" kind of freedom - big, heavy, nation-building, humanity-saving freedom).