There are two main types of zombies:
Living Dead - The original zombie, they are 100% dead and therefore hard to kill unless you go for the head. This strength is offset having to infect and turn the victim into one of them, which takes time, up to hours. Zombie Survival guide says as much, the brain needs time to metamorphosize into a new organ capable of functioning without oxygen. (as seen in Night of the Living Dead, Dawn of the Dead, Dawn of the Dead remake new with faster zombies, and the World War Z book) Note that they shortened the time for the change in the remake, but it was still minutes.
Rage Virus - Here's your more Hollywood zombie created for the silver screen. They're faster, more ravenous and scarier since the infection takes seconds instead of minutes. This allows them to wash over living populations like a tide. But they aren't dead, they're just infected berserkers. There's no time to transform them into living dead. Sure they feel no pain but just about anything that'll kill a living human will kill them. On top of that, being alive and uninterested in eating, they'll eventually starve to death. (as seen in 28 days/weeks later, Left 4 Dead)
Now, watching this movie, the zombies behave almost entirely as your standard Rage Virus infected. They're uninterested in consuming, only infecting. They spasm around for mere seconds after being bit before becoming one of the horde.
And yet they're not satisfied with that. These have to be dead too! They mention a headshot is the only way to really put them down. Corpses still move after being torched. Virologists say they can't use diseases against a dead host.
They had a decent zombie type to work with, why did they have to be dead too? It had no bearing on the movie, except to explain why injecting them with toxins wouldn't work. No, they had to be the SCARIEST zombies, a combination of the scariest parts of all zombies like some kind of.... franken-zombie, even if it didn't make sense.