I know the title question sounds somewhat convoluted, however it is an important question both from an ethical perspective and from a logical one. I will address the logical part here. It is not uncommon in school, that a student approaches another and asks to look at or copy the homework assignment. It is possible that the student did not have time to finish the homework, thinks s/he got the answers wrong, or just forgot to do the homework. Regardless of the reason, the bottom line is, it is not completed and it is due that day. So what is one to do? A common approach is, s/he asks a friend for help. Once in a while most people do not mind helping out a friend in a rut. 
What if anything is one to do though when the teacher catches you in the act of checking or copying your answers from your friend's homework? Is there anything that should be done? Should a teacher interfere? Should a teacher punish one or both parties?
In one incident, the teacher gave an "incomplete" on all students's homework assignment, that is the person that supplied the homework and the persons that were using it to complete his homework.
From a parent's point of view, I have to disagree with the teacher. I don't see the value in punishing the student that completed the homework, put in the time and effort to learn and practice. While I understand that copying homework assignments could undermine the student's learning of the material. But giving outright an "incomplete" grade instead of looking at the situation in its unique light and find the reason or reasons why there was a need to use another's homework answers does not serve a better purpose.
Giving an "incomplete" on a completed homework for "allowing another student to copy answers" is outright illogical. If it is complete, acknowledge it. What message will it send to the students if the teacher "lies" in the grading about the homework assignment. Has the teacher given a warning to the student or even told the students to not let anyone copy or check their answers? If one compares this to copyright infringement, no one ever punishes the copyright holder that was infringed. And there are plenty times when there is consent to use a material in one way but not another. Similarly, one may allow one student to check answers, and not another to copy them, yet still happens. So in my mind, there is no logical reason to punish the person with the answers. As for the student that copies them, it should be judged on a case by case basis. 
From a teacher's perspective she gave the student an incomplete instead of a zero due to that fact that she doesn't believe the student had any intention of allowing her paper to be passed to another student. She had discussed the situation with another teacher and the impression she got was that the students were not working together, rather copying work from each other. The teacher feels that it is a learning experience for all involved. There were two points she thought were important to be learned from this situation: "1. homework should be done at home and during appropriate times in school, not during instructional time; 2. homework is for students to practice skills and to discover the areas they need to review."
While the teacher may have a point or two, they only apply and address the students who did not have their homework done. The teacher's "lessons" have zero value and message to the student that did the homework at home, and thus practiced the skills. Again, it seems illogical and makes zero sense to me. I can only imagine how confused the students are.