For example, a student of mine once proposed writing about No Child Left Behind. Granted this isn't as sensitive a topic as those listed above, but it's still significant. I emailed the student saying if they were to write about No Child Left Behind, they would need to start by reading the law itself. I provided a link to the 670 page document.
And No Child Left Behind is just a simple example. Doing gun control, abortion, or gay marriage would require careful analysis of decades of legislation, court cases; and then accompanying sociological, psychological, biological, and chemical research, before diving into the world of ethics. To complicate matters, the myriad of opinion pieces on such topics found in newspapers, magazines, and blogs are highly subjective and therefore questionable, resulting in a continuation of Confirmation Bias on any side of the issue. So, properly addressing one of these topics would require hundreds, if not thousands, of pages of research; not all of it reliable, spanning decades of both primary and secondary sources, to be in a situation to authoritatively say something about it. Not many freshmen feel like putting that much effort into three pages.
When you consider the scope of research, this actually broadens inappropriate topics into less controversial areas, like childhood obesity and organic farming, and even more mundane, less controversial areas. Let me share one of my favorite examples. A colleague of mine said that a student of hers wanted to prove who Jack the Ripper was... in a freshman composition course. At the end of the semester but before the final essays were due, my colleague said she asked the student who Jack the Ripper was. The student hadn't come to a conclusion.
Personally, I’m not a criminologist, but I don't think there's a branch of anything that hasn't contributed something to the resolution of this Victorian murder mystery. Psychology, sociology, biology, chemistry, even literature has offered conjectures. There have been hundreds of books on the matter, and, I'm sure, many more articles, all of it ranging from the fanciful to the serious. To definitively prove the identity of Jack the Ripper in a matter of weeks would be to try and do what hundreds or thousands of experts have tried to do for more than a century.
Whether or not a topic is or is not appropriate should be determined by the structure of the class. Some topics, because of their scope and the research it would take to handle them, go far beyond the demands of a general education composition course. Education should be challenging and students should rise to that challenge, but it can be far too easy to get yourself in too deep.
I want my students to push and challenge themselves. I want them to branch out, learn, and grow; but my students are college undergraduates taking a course that is designed to give them writing tools for other courses. Many haven't settled on what to major in and are taking three or four courses in addition to mine, as well as their own social lives, jobs, and family obligations. I want to challenge my students; not set them up for failure.
Returning to my colleagues who encouraged students to write about controversial issues. After a year of teaching, pretty much all of them had changed their tune: they forbid their students from such topics. They hadn't undergone a moral or ethical change, but they had a better understanding of what their students could handle.