The question Good uses of computer viruses was closed as off-topic, and is being reopened as I write.
Is this question on-topic on the Computer Science site? Why or why not?
I think the question is of high quality, it it certainly connected to computers and there is a lot of computer science connected to viruses. However, even if the topic is connected to Computer Science, that doesn't mean every question related to the topic also is.
The first paragraphs describe some business concerns and some IT concerns. The OP mentions two alternatives, the first of which is a user experience question and the second is a legal issue (I'm pretty sure it's illegal). The OP then finishes with a rather broad question.
Even though evidently a lot of people found the question interesting and good (even I thought so), I don't think the question should be on-topic on this site: it may be a very good question on some other Stack Exchange site.
My problem with the question is that there is no 'science' involved. There is no theoretical framework behind the question, but only a question for 'good' uses of viruses. There is no definition of 'good', nor any indication what the OP is looking for.
All the answers talk about ethics, legal issues, networking implementation details, specific flaws that might be exploited and specific things that particular examples of viruses have done in the past, but here again I see no theory or science involved.
Now, this is all according to my personal definition of Computer Science, and I just offer my view on the matter. The scope of the site is far from set in stone right now, so this meta discussion is very useful.
Based on comments under the question, there seems to be several interpretations of what is being asked. This shows that the question is not well-defined at the moment. If the OP edits it and makes it clear what he is looking for then it can be reopened. At the moment it should be closed as "not-a-real-question".
I think questions about viruses should be considered in scope, especially if we see it as either a technical or thought experiment kind of question.
As to the issue of whether there is any science involved:
I don't think we should be ruling out too many questions at this stage, especially since trivial things such as Are Turing machines more powerful than pushdown automata? pass without comment.
 Simon Kramer and Julian C. Bradfield. JOURNAL IN COMPUTER VIROLOGY. Volume 6, Number 2, 105-114.
The question was closed with a reason as "off-topic" instead of "too vague". This is the first thing wrong.
Second it was a was a good question and was getting good answers anyway, so why close? The reason to close a question as vague or overly broad is because it is hard to tell what is being asked or because the answer may be everything, and this makes the question useless. That types of questions are harmful to stackexchange sites because they tend to create noise. However, that was not the case here. Even if the question was worded as broad, it still was a good question getting good answers, so should not be closed.
If you think the question should be improved, suggest how to do so without creating artificial restrictions and restricting it usefulness just to say that it is not broad anymore. If you can't do that (and don't think that the question is a unsavable doomed garbage), don't close it. In my opinion, it is already good in the way it is now, so I keep my vote for reopening.