But this is precisely why it makes this system useful AND should please teachers. If the system works well for grammar and ( hopefully programmable) essay structure, then the teachers can focus on the content, style, and finer points of writing. A computer can correct to/too/two, and if it frees up more time for the expert that is being paid to grade in depth, this is a good thing.
That is exactly what my last 3 English teachers did. They would mark improper grammar and spelling errors, but generally did not care as long as the content was effectively presented. These courses were all generally above the required university-level writing courses though, so it wasn't like we were turning in a bunch of youtube comments. At that point, our teachers let us into the secret of English writing...grammar really doesn't exist as long as your writing is understandable.
My theatre teacher took a different approach though and would make us re-write a paper that had grammatical errors. He also did not point out what they were. In those instances, we had to rely on our classmates to check our work.