Why divide movies left and right?

My last couple of posts have dealt with conservative and/or liberal movies. I find the intersection of movies and political philosophy interesting. And I found the sneering tone of a Chicago Sun-Times editor interesting too. The last post brought about this comment from a good friend of mine who is actively involved in the movie industry:

I'm still confused as to why movies have to be classified as liberal or conservative. Granted - a movie MAY be that way, but why do they have to be politically divided from the start? I'm not trying to be snarky, I'm just really actually curious. I guess I've just never thought of them that way before (okay...maybe there are a few political satires which obviously fall one way or another but just normal run-of-the-mill-movies? Am I missing something?)

This is something I've thought about over the last week or so and I can think of three reasons:

One, a political commentator may want to act as a guide to others. Imagine a couple with kids who have few oppurtunities to go out and see movies. They think that the ads for Movie X look good. They get a babysitter and go. And then find out that the movie's viewpoint is frustratingly slanted. Knowing ahead of time could have saved them.
An example where this could help is 'Runaway Jury'. The story is about a court case involving the gun industry and whether they should be liable for gun deaths. The movie presents the 'of course they should' case, without ever showing the opposite. Knowing that ahead of time could be helpful. (I think that's the spirit of the first article I linked.)
The second reason would be to showcase the bias of the movie makers. One of the arguments from the right (which I agree with) is that popular culture is overwhelmingly presented from the left. Liberal arguments are shown as mainstream, while conservatives are shown as god-bothering crackpots. Pointing out that a movie doesn't come from a neutral standpoint is one way of leveling the playing field.
The third reason is the one I'm most comfortable with. Conservative intellectuals are very introspective about conservatism. Movies with philosophical heft give us oppurtunity to refine and celebrate our beliefs. Being able to point to a movie and say "this shows what I believe" is amazingly satisfying. I'm sure the same thing holds true from the left.