I wouldn't describe the Romero films as simplistically as that. It can be debated whether or not Night of the Living Dead has overt themes about race relations all day, but the themes about consumerism in Dawn of the Dead, conservative government in Day of the Dead, and the age of Youtube in Diary of the Dead make those films a little more complex than the formula they use to accomplish their messages.
Yeah, but that's not what I was getting at. Just about every zombie movie sticks to a plot formula. Rarely have I ever seen any zombie film not break the formula that I mentioned. Zombieland was different in that it broke the formula.