That would only be true if the question was a multiple choice one.
No it doesn't. If the question is "Write a Haiku about C9" then a valid answer is
Maddus on C9 Is often controversial But fun none-the-less
Now suppose that haiku gets 100%. Now one of the students writes the answer on their blog. Another student looks at their exam paper: "Write a haiku about C9" and they type it into Google:
Ah! I've found one! And copy the above haiku onto their exam paper.
The teacher receives 30 copies of the same haiku as the answer to the exam question. Now since the exam is deterministic, all of the children get the same mark, and even though it's obvious that they all cheated, we're specifically allowing cheating, so we can't deduct marks for that.
So of the 30 students, one wrote a good answer to the question. 29 just googled the question, wrote it down and got full marks.