The claim is that writing can't be taught on an industrial scale but science can be.
No, these classes are taught this way out of necessity. If a grad student wants a physics degree, (s)he must do research. To this end, a typical physics grad student will TA for just a few semesters until (s)he gets a research project. This is a full time job. There just aren't enough incoming grad students to fill more teaching positions. I suspect it's similar for all the sciences.
I don't know what it's like for English grad students, but I suspect that it's the exact opposite. As far as I'm aware, English majors don't need to do the same kind of time intensive research. I suspect that English grad students will teach classes throughout their higher education.
Thus, even if there are the same number of incoming physics and English grad students, I suspect that there will be 4-6 times as many English majors available for teaching.
To err is human, to moo bovine.