Teachers are under a lot of pressure to be "tolerant of different means of self-expression." This means, sometimes, not criticizing students for grammar, spelling, illegible handwriting, or other examples of non-conformity.
I can agree with this, to a point. I would hate to see a student pour out his/her heart in impassioned, and well-reasoned, prose, only to have it returned with "You split your infinitives - C+". On the other hand, it is a disservice to students who will have to write professionally not to teach them how to write in an accepted format. Unfortunately, a lot of graduates hit the working world with the attitude "as long as you know what I meant, it's fine."