I don't get it. Isn't that what people wanted to happen? Don't produce copyrighted work in the output. Sure you can learn from it, much like a director might learn from hundreds of movies he's watched. He obviously can't copy the plot from Die Hard but he can use elements he's picked up from it to make a Christmas movie.
What do you mean he can't copy the plot? Any commercially successful movie is using one of about five basic plots.
While the number of plot structures are fairly countable, a plot can still be considered different by changing part of its contents (character, setting, etc.). The decision on whether a plot outright infringes on an existing plot varies case-by-case. An example of outright copying is "Fistful of Dollars" directed by Sergio Leone, which lifts the plot from "Yojinbo" by Akira Kurosawa.