Frank on the beach

Porty seemed to be very fortunate with the weather, with next to no rain and merely a slightly blustery day on the beach blowing the sand about a bit. Unlike many other parts of the country that seemed to have a very wet time of it.
Later in the evening watched the first of The Hobbit films on TV. Having never seen it in the cinema seeing it in the schedules I was interested to see what they had done with one of my favourite childhood books. In particular the scene with the trolls very early in the story. I still remember it being read to our class at primary school, and have a memory of the teacher doing a great job with the voices. Anyway I'm not sure why the narrative in the book needed changed so much for the film - why did Bilbo and the dwarves not just see the light and go and investigate as described in the book but instead were seeing what had happened to some missing ponies? Why did Bilbo not try to steal a magical purse that shouted to warn its owner? Why did the dwarves not arrive one by one and get bundled into sacks and then Gandalf returned secretly to get the trolls arguing until the sun came up? That was the conclusion to the scene, but only when Bilbo had caused the argument and then Gandalf cracked a massive rock to reveal the sun. And while I'm at it, in the book Gandalf had to go back to get Bilbo to set off on the quest at all as he was just having breakfast after the late night with the dwarves. In the film, Bilbo found a note and decided to go himself. Why on earth change these aspects of the narrative? Obviously a film has to select elements of a book to fit into the shorter form but if you include a scene anyway, why change it when leaving it as it was would be grand? Is it to give screenwriters something to do when what they should be doing is simply adapting the text for the screen?