I was skeptical at first, but now that I think about it more, I am okay with this. A TON of stuff goes on behind the scenes in the Hobbit. But the book is written from Bilbo's perspective and he really didn't fully understand all of what was going on at first. Neither did Tolkein; I know he revised the Hobbit later to make more sense as he was writing The Lord of the Rings. In the original version, when Bilbo and Gollum were playing the riddle game, Gollum willingly bet his magic ring, which would not make sense in the context of LotR, so he changed it to Bilbo finding it and basically stealing it from Gollum.
Gandalf went off and did a bunch of stuff that wasn't explicitly mentioned in the book. He met with the other wizards and the elves, attacked Dol Guldur, and drove Sauron ("The Necromancer" as he was called in the book) away to Mordor.
Basically I think Jackson realizes that he'll only get so many opportunities to make movies based on Tolkein's material, and a Silmarillion movie is probably a long shot. So why not throw in as much backstory and side plots into The Hobbit as possible? I mean, the LotR movies came out 10 years ago, before the more recent trend of dragging film series out for 4, 5, 6, or even more movies. If he was making LotR today, who knows? He may have convinced the studio to go for 6 movies, 2 per book.