The changes are done for the flow of the story. I think this piece sums it up pretty well:
io9 GoT story

The whole thing with Theon is a pretty big change, I am curious as to where that is going. I mean, a lot of this is done to show "concise" character arcs for TV. I used quotes cause we're still following like 20 characters, but you know what I mean. Shoot, if it was up to me, I'd just follow ONE character for the entire episode and have a time line a-la 24 style to show how the story is progressing.

shoot, if they followed the books, season 4 would pull a lost and have an entirely new cast on the other side of the island! and if that's a Lost Spoiler, you'll thank me for not disappointing you like the ending of that show. /lostfanburn!