This is an interesing incident that is relevant to OCW publication. The argument of the professor is that students posting assignment answers after the due date are somehow facilitating cheating. The assumption is that the instructor was going to recycle the assignment in a later semester, and this would allow students in subsequent classes to cheat.
This strikes me as being somewhat naieve on the part of the instructor. The fraternities at MIT are widely known to keep assembled bibles of answers to problem sets and other assignemnts (see “Don’t work from scratch” here), and I’d wager the same goes on at most other univerisites. In some ways, open posting like this is more democratic, leveing the playing field.
It also shows why teachers really do need to change up assignments regularly. I had a geology prof at West Virginia University who would distribute his multiple choice tests ahead of time and would tell us that these are the questions we would receive, except with a word or two changed in each, which woud change the correct answer. A very effective method, as it forced you to understnad the concepts.