What is the policy here on homework?
Why do Stack Overflow users close questions? Shouldn't we be encouraging homework questions, and encouraging users to ask homework questions in an appropriate manner?
I think that homework questions ought not to be discouraged, but rather guided by a basic principle: The poster should have made a good faith effort at the solution. This is generally easily discernible in a question; those that have only a plain question and no subsequent explanations of attempts (e.g., "I was thinking that…" or "What I tried was…") have obviously either not been attempted or haven't been attempted seriously. I think questions like this warrant at the very least to be ignored by the SO community at its discretion. They might, as you ask, even deserve closure as well.
For example, if a user posts something like 'What is O(f(n))?' I think it is pretty obvious this is a homework question. Should it be treated any differently than any other question? Based on my point above, I would say so. In my opinion, it is always better to post a question that leads you to an understanding of the concept, rather than a specific answer for one instance. In this case, a question along the lines of "What is Big O?" obviously stands to teach the OP a lot more than the original version. In the case of the first version, it's some pretty easy rep for someone to post 'O(n)' or whatever it is, but answers like this only stand to encourage the exploitation of SO, and so we can argue that they might in fact warrant closure.
But again, the fact that a question is tagged 'homework' or appears to be such should not warrant its immediate closure. I think a question like 'What is O(f(n))?' is completely valid if the user has a particular question that he has genuinely attempted and cannot solve. He might not understand some less-than-obvious concept, or he may have overlooked it. These questions should be encouraged because chances are if he does not understand it, there will be someone in the future who is in the same situation and will benefit greatly from the fact that the original question was not closed outright.
I'll finish with some food for thought: SO is supposed to be a place for questions "that can be answered." Do they come any more answerable than these simple homework questions?
I agree with what you say, but in practice the kind of OP who requests (or demands) homework information, is never heard from again.
At least the comments which ask for more information (like "show us what you've done") never get a reply.
I think some SO users have developed an ability to recognise this kind of post, and close it ASAP.