I have heard people say that John Williams stole this or that, but there is a HUGE difference - his main themes were completely original and totally identifiable as his own music. There is no classical composer who created Star Wars or Indiana Jones or Schindler's List or Superman melodies and then John Williams just disgusied them - those are pure John Williams and his own very original style.
https://www.youtube.com/watch?v=V47enEvsafQ
Not to mention the well-known similarity with "The Planets", but also a couple of musical ideas stolen from Stravinsky. And I'm talking just about Star Wars..
I perfectly agree that composers for movies have to start from something that is already in the mind of the audience, because as a composer you have to speak with a language the listener can understand and relate to. But sometimes as Williams says, it almost becomes a carbon copy. Personally, I would be just fine if they would avoid to award those composers with an oscar, if the soundtrack is clearly "inspired" from the classic repertoire..