If I am writing a novel and I copy a section verbatim from another novel, I am infringing on the other novelist's copyright, regardless of whether I wrote it from memory or not.

And this makes sense. For a trivial operation, there might be only one way to write the code. That's not copyright infringement, just like you're not infringing on an author's copyright by occasionally writing a sentence that was similar to theirs. For a nontrivial operation, you can easily write your own code without copying someone else's work.

Remember also that you can use others' ideas. Copyright only cares about the code itself. If there's a clever trick that you've seen someone use, you're free to use the same clever trick as long as 1) they didn't patent it and 2) you're not actually copying their code