The reason for it saying "for some reason", is because that is one thing that does vary from movie/book to movie/book. Sometimes, it bio-warfare that infects them, sometimes it's moon rocks, sometimes, it's just unexplained and characters believe it's Revelations with the dead walking the Earth. The rest of the Zombie are fairly consistent in modern Zombie stories (Must damage the brain to kill, bites or scratches infect you, etc.). About the only other important rule that varies is wether you turn into a Zombie upon natural death, versus being infected, and typically a natural death will also give you Zombie-itis in modern Zombie movies
I get that, it's just that most of these stories try and keep to some sort of 'reality' rather than imagining their own details. Like zombies with toxic blood, for example. Where'd the zombies get that, where'd they get the new parts that can HANDLE toxic blood?
Silly to try and use that to refute more generic points, as it's obviously specific to one case, and doesn't fit with MOST examples of the generic zombie.
Sure, they're all slightly different, but if you want to discuss them in general, best to start from a basis in reality (ie the human body it starts from) and not try and start with a magical space zombie. His toxic blood defense is just as valid as refuting the point by saying that zombies can fly