There's a difference between writing a set of variations on a composer's theme (which is then titled accordingly as: Variations on a theme by [insert name here]) and borrowing music which is then not credited.
If a person wins an award for best 'original' soundtrack - as so many of these film composers have done - whose music is really getting the award? On the Wikipedia page for Girl with the Pearl Earring, it talks about Alexandre Desplat 'creating a melody that recurs throughout'.. except it's not really his melody at all it's Mozart's, from the 2nd movement of his F major piano sonata. Many scores nowadays are pretty minimal, based around a theme which is basically repeated (e.g. Yared's Lives of Others; Desplat's dreadful score for the Painted Veil), so all they have to do really is devise this one theme. All or a lot of the actual orchestration is done by others anyway in so many cases.

Borrowing elements is one thing, but lifting a theme or melody and using throughout, especially without credit, it theft or at the very least deception.

> Well I don't think he stole music, been greatly influenced by those u
> composers would be the right words. Who could blame him? Composers of all
> time were influenced by previous composers, if not writing variations upon
> other's. Note the Beethoven and Mozart's classical style in Chopin
> romantics works. Like the other guy saying that the Qui-Gonn's funeral
> scene and 'Duel of the Fates' were copied on Orff's Carmina Burana: of
> course, a choir on a funeral scene would have this tone, like Mozart's
> 'Lacrymosa'. And for 'O Fortuna', a lot of people thought that it was used
> integrally in 'Duel of the Fates', a mistake for people's lack of musical
> earing. Of course, I'm 17 and I don't have the knowledge of a major in
> music composition student, but I think that those 'stealings' are simple
> influences.