Fiction does not excuse lapses in logic based on the rules of the world the story is set in. That is why there are so many websites set up for movie mistakes and such, since it tends to bug people when the rules of the world change or are not followed. The rule they established is that these people are dead, and that they have decomposed enough within the first season to be stinky beyond belief. They never said anything to make us change the way we think of normal biology, in fact they gave us enough medical information to reinforce the regular assumptions regarding biology, to wit that the bodies are dead and decomposing just like a body does when it is dead, but it is mobile during the decomposing process. They show body parts falling off and being pulled off to reinforce this as well. If anyone has dealt with a dead body in any way that wasn't preserved somehow the stinking rotting phase can last from as little as a month to about a year (again depending on conditions the body is exposed to) in which time it decays enough to be a skeleton more than a "body", and would lack enough connective tissue to approximate any kind of locomotion. These bodies are going on 3 years decaying, and mostly in optimal conditions to increase the rate of decay.
I am just saying that I would like to see them come up with a plausible solution as to why, so help maintain the gritty reality of the show so it doesn't devolve into a caricature of itself as we get into season 6 or 7 with the same dead bodies running around for 5 and 6 years which is just flat-out impossible. Like I said at least a passing nod to the logic of the world they have created, or a plausible reason to ignore that logic.