There are a fair number of legal issues related to music. Examples are downloading copyrighted songs and performing music you obtained "unofficially".
Should questions about legal issues be allowed?
I think these questions should be off-topic.
Questions about legal issues are either too localized (asking about laws in a specific area) or too broad (we can't answer well without knowing your specific area), and rely on legal expertise and not musical expertise. Most of these issues are only tangentially related to music; downloading a book illegally or reading it aloud in public have largely the same implications as the issues mentioned above in the question.
While answering these questions could be helpful to musicians, I don't think they are questions about musical practice, performance, theory, or history (the scope of the site). The scope should not be expanded to include them because we are music experts / enthusiasts, not legal experts. There may also be legal ramifications of giving advice on legal issues, which we don't want to deal with.
I've made a more expansive post on this subject for Gaming, which I think largely applies here as well.
I think these questions should be permitted and on-topic. There is a great deal of bad information out there on this vital subject, particularly among the younger generation, who tend to be scofflaws. As a musician, I think it's important to uphold the law and to educate others about what the law is and how to apply it. I try to discourage piracy and theft of intellectual property wherever I see it, and to educate others about combatting piracy and theft. The laws exist to provide a framework for musicians to earn a living, and Internet piracy has decimated the incomes of so many musicians that I know and work with.
I'm certain that my post here will be unpopular, but I've registered my opinion on the subject.
And, in the course of answering questions on Music.SE, whenever I see this sort of thing come up, I'll write about it.