Explain the current, conventional “interpretation” of Soviet films that “Sabina always rebelled against.” Why did she take a position contrary to popular belief? Why is this significant?

Kundera's fiction is a critique of the cultural attitudes communism imposes upon its people. In his novels we are presented with characters who either resist the herd-mentality of the Eastern Bloc countries or who are simply unable to understand it and make naive "mistakes" in reaction to it, as the character Ludvik does in Kundera's earlier novel The Joke, where a light-hearted remark lands him in prison.
In The Unbearable Lightness of Being Sabina is an artist herself. But she is a rebel, uninterested in responding in the accepted way to the kind of substandard art that is promoted in the Soviet Union and its satellite countries. Soviet filmmakers and others in Eastern Bloc countries were usually not permitted to express artistic themes independently of the Communist ideology that pervaded every aspect of life. It is not merely the films themselves, but the way they are to be interpreted, that the Party seeks to control and which Sabina's unorthodox mindset rejects.
Even a Soviet film which is artistically on the highest level, such as Sergei Bondarchuk's 1968 War and Peace, for example, though there is literally nothing in it having to do with Communism, would have to be interpreted by a dutiful citizen as if it did have a Communist theme. This is because the Party demands that all legitimate art have such a message. This conformist mentality, of course, is the very thing that Sabina resists.