So the question is not, exactly, “Why change the books?” Because the answer is clear: Many, many details must be changed, just to make the transition from book series to televised series work. The question is, instead: “Why change this?” Why make a scene from the book that depicts consensual sex into one in the show that depicts rape?
Thank god for the A.V. Club, the only media outlet that has outright called bullshit on this so far.
And another really important quote:
Rape is a tricky thing to use as character development, for either the victim or the rapist; doing it twice raises a lot of red flags. It assumes that rape between characters doesn’t fundamentally change the rest of their story—and it assumes that the difference between consent and rape is, to use the parlance, a “blurred line.”