I hated Avatar 3D for that reason. They still hadn't figured out focus (human brain, not lens)...
I did some more reading after I posted and you are right... the blurriness of backgrounds is the human brain's inability to adapt to the projected 3D. But still, if the blurry effect is there for most people [including yourself] then I think my point stands that it's limitation of 3D.
For a more recent example than Avatar, I saw Star Wars TFA in I-max 3D and regular theater 2D. TFA has some cool 3D scenes, but scenes that had stuff in the background, like Maz's Cantina and large battle scenes, were way better in 2D, because I could see everything on the screen clearly. Overall I preferred the regular 2D version, because I like seeing all scenes clearly over the few good 3D effects. Also when I can't bring a background image into focus it breaks my suspension of disbelief in a movie.
I am also concerned that if theater and home 3D becomes more common directors may not bother applying much creativity to backgrounds because 3D viewers probably won't notice it anyway. Technology should enhance the artistic abilities a movie director, not place artificial limitations on it.