Look, I love the books. But his past 2 books, if considered a TV show unto themselves, would be 80% fluff and 20% actual core story. You know, the kind of TV shows you guys all bitch about.
You have 10 episodes a season. You have actors and production staff you need to keep working. These people need to eat. Sometimes actors die. Kid actors get older. Sets cost money to maintain. You don't have the luxury of 5+ year gaps between seasons unless you're cool with the idea of the cast changing every season.
GRRM has given them the core skeletal structure for the unwritten books. That is HUGE. While it is the little nuances that can make a story enjoyable, any writer worth their salt will outline the big shit first and then work on stitching it together after.
It is unfair to lump Tru Blood in with this, primarily because the source material itself was pretty shitty (if they had remained true to the books, the show would have been even more WTF in my opinion). The show runners have shown that they are able to make some good improvements. Remember the whole Arya and Tywin bits in Harrenhall? Completely made up, but completely awesome. The expanded Hound + Arya stuff? Hell, even the Red Wedding adjustments were good ones IMO, because it made the scene more brutal (killing a pregnant woman and Frey allowing his young wife to die did much to make him even more evil, as opposed to no pregnant wife death and some random mentally challenged Frey dying). Sometimes media can improve on the source material. King thought Darabont's ending to 'The Mist' was ballsy and how he wanted to end it, but couldn't make himself write something that damned bleak at the time.