Stallman's ideology is actually fundamentally libertarian. The core principle of libertarianism is, "Your rights end where mine begin." It doesn't matter if you're a state, a corporation or another individual. Your rights end where mine begin.
Amongst the rights granted to each individual is the right to modify the tools that they use. You restricting my freedom to modify is as wrong (according to Stallman) as you restricting my freedom to move about.
I think "libertarian" is close but not quite right (unless you are coming from Chomsky's viewpoint, which is a niche within a niche, at least as far as US libertarian thinking is concerned).
If you take libertarianism to mean "allow anything that is consensual and does not involve externalities", then proprietary software as implemented today is totally fine. The user consents to an agreement that amounts to "I'll give you the output of my compiler, but I'm not going to give you the input. Also, you have to promise not to redistribute it."
Of course, setting up the details of that arrangement for every creative work would be a huge hassle, and copyright is an attempt (in my opinion a failed one) to streamline it. One way to envision copyright in a libertarian framework is a legal presupposition that when I tell you a "creative secret" (copyrighted work), you'll keep it secret (won't redistribute) unless I give you explicit permission (a license).
*Edit: to elaborate on why I think copyright is a failed streamlining of the above framework, I'll just give one example. If I broadcast my "creative secret" out loud (or over the EM spectrum) then I can't reasonably expect it to be secret. The people who listen to it and hear my "secret" have never formed a consensual agreement with me. Thus, copyright as it stands gets twisted into "you can't participate in public communications without promising to pretend that various things you hear are owned secrets"
Amongst the rights granted to each individual is the right to modify the tools that they use. You restricting my freedom to modify is as wrong (according to Stallman) as you restricting my freedom to move about.