As a Ph.D. student, I have TA'd many classes, and it seems every semester, we have caught at least one person for directly copying homework answers from a previous year's solutions. In most cases (probably 8/10), the instructor does not want to begin any formal academic integrity hearings, so all that happens is the student gets a 0 on the question with no other punishment.
It doesn't seem fair to me that there is no formal reprimand for cheating other than getting a 0 on the question. When I have asked instructors about this, their reasoning is that creating a formal hearing/meeting is too much work, and they often add that the students (who are overwhelmingly international students) may not understand that directly copying solutions is not acceptable.
The question is, if giving a 0 on a homework question an adequate and fair punishment for cheating? Why don't the instructors want to take any additional steps?
Somewhat related: Professor does not care about cheating, what should TA do?