The difference is art vs propaganda. The "message films" of the 70s flirted heavily around the line and got down right anvilicous about it. Indeed, losing the quality of being entertaining. A bit of film might have something to say, but it should never overwhelm the film itself.
This is why Star Wars was the blockbuster it was. Pure Space Opera Hero's Quest with no lesson or message it had to teach. It was cold spring water to a thirsty population. People that did not like Science Fiction went to see it, repeatedly.
