In this film, Richard Stallman presents his ideas very directly in interviews, so any left-wing bias is not really "hidden." This is a good way to learn about his views.
I meant more towards the very anti large non free software business messages that are in it.
For example (all microsoft examples here off the top of my head)
Raymond giving the thousand yard stare at Microsoft exec
Bill Gates open letter
Richard talking about MS being a small player initially
The Microsoft free software refund meeting
All were made large closed source businesses look like the enemy. So the show is very biased especially towards Microsoft. Richard is indeed very open but the movie can be seen as a propaganda piece rather then purely factual.
Americans and Europeans mean different things by "left wing" and "right wing." In Europe, the spectrum goes from socialists to fascists. In America, the spectrum is much more narrow. It goes from a highly regulated state to a highly deregulated state, speaking economically. So in America, being against huge corporations is basically a left wing thing.
The free/proprietary dichotomy is not the same as the left/right (or communist/capitalist) dichotomy. They're mostly orthogonal, in fact. Unless having any kind of ethic makes you left wing, but I doubt you think that.
Also, if the film doesn't look balanced, that's probably because the facts aren't. But to the point of looking like a mere propaganda piece? Only the background music gave me that impression.
It is however very left wing, and fails to mention any flavor of BSD but you can't have it all.