It has a lot to do with ability to see where you are going and hence predict/be aware of the vehicles movements. To put the extent of this in perspective, I am noticibly more likely to feel motion sick sitting in the back seat compared to sitting in the front seat. This effect is reduced by sitting in the middle, where there are no seats/people obscuring your view. Additionally sitting in any seat except the drivers seat while travelling on windy mountain roads is a surefire way to bring up lunch. Reading while in a car is completely out of the question.
Comfort should be a consideration of any type of transport and the thought of facing backwards in a car makes me uncomfortable. While this shouldn't outweigh safety, it certainly shouldn't be ignored either.