It is an inconsistency between Walking Dead and other zombie movies, but not one I think exists in The Walking Dead itself. I dont think anyone was bitten and turned right away. When Andrea's sister was hurt by a zombie, she was dead for a while before she turned. So its reasonable that being bitten isnt a death sentence in itself.
As to why they eat...well, why are dead people walking around? Normal human physiology doesnt apply to zombies. They are more supernatural. A virus doesnt explain their condition. Eating is just something they do. In the first episode, Jones's (the father) wife seemed to exhibit memories of her life, because she kept trying to get into her house. In Atlanta, you saw people waiting on the bus like they were going to work. Maybe they eat because they remember instinctively its something they do. Three things that all humans do is eat, shit and sleep. The walker that got Hershel might have been sleeping. Otherwise, that was one sneaky zombie. :biggrin: