I think it is has become an airborne plague that when you die, you rise, OR you are bitten. Many of the books in the genre discuss just that. World War Z is exactly that way.
My bet is this is just a tie in so they can pimp for money when WWZ goes live.
Either way, I am glad Shane is gone. It clears a storyline that received far too much attention and in reality, my wife agrees. She says no effing way would she hop into the bed of my partner especially with all the hell on earth. It just made little sense. Have to write a story I guess.
I do enjoy the series, but I fear it is getting into too much drama and too little reality of the situation. Times a ticking, and these people have done little to look at real long term survival questions. That alone is something that begs to be talked about.