If you are talking about much of the extra material in the move, that, in fact, uses Tolkien's work as its source material. For instance, the story of the battle at Moria's gate and how Thorin earned the name "Oakenshield" is part of Tolkien's history of Middle Earth.
The meeting where Gandolf, expresses his concerns over the Necromancer, while not mentioned in "The Hobbit", did take place at during that time of the story according to Tolkien. In fact, this is the reason that he gives for Gandolf being absent for a good part of the book; He, and rest of the White Council have gone off to deal with that threat.
So essentially, What Jackson is doing is integrating in events that happened during the time of "Hobbit" in order to tell the larger story. Setting the dwarves and Bilbo off to deal with Smaug was in fact just one part of a plan being executed by Gandolf.
I'm not saying that Jackson hasn't taken some liberties (for instance Azog, the white Goblin was killed at the Battle of Azanulbizar, and it Gandolf and not Radagast that goes to Dul Gulder.), But I think I can understand why some of these changes were made for the film. (He did the same thing with the "Lord of the Rings"; Sometimes having one character do something that was done by another in the books.)
