From what I gather, whether students use Physics.SE to cheat or not has never been our primary concern when considering the homework policy.
The main reason behind the "harsh" homework policy is that the SE is not a forum where nice people come around and solve your problems for you. It is intended to be a community of more or less knowledgeable physicists answering each other's questions so that other people asking themselves the same thing may learn from it.
We are not primarily about helping the OP, though a heartfelt thank you can make the day of an answerer. We are about answering questions in a way that might be useful to other physicists or interested laypersons as well, and solving problem No. 34.23(iii) from some book is not going to do that. That's also the reason for the "harsh" requirement that an allowed homework question should ask about a conceptual problem, not the solution as such.