Well, essentially, M and P are trying to scramble up a stoney place to get a better look around and from what I understand, Treebeard is standing at the foot of it. Perhaps PJ had some difficulty finding just the right sort of place for this scene and just had the young hobbits climb up Treebeard. I don't know, but I bet it's ok.
I don't mind the scene, because the cover of my old '70s book has a watercolor done by Tolkien, showing M and P at the base of a "tree" with very startled eyes and one of the hobbits is scrambling up the roots of this tree. I always thought it was Pippin, because the other is lying face down, as if recovering from exhaustion.
I don't really mind it...I'm just saying it's different. I hope that in TTT, since they got most of the background stuff out of the way in FOTR, they can put more of the "book stuff" in there. I'll be happy if they get the scene at the gates of Isengard right.