So why isn't it acceptable to submit the same paper for multiple courses?

From The Ethicist:

When I was in college, I’d sometimes write a single paper that would satisfy assignments in more than one course. For instance, I once wrote a paper on how “The Love Song of J. Alfred Prufrock” expressed satire; I submitted it for assignments in both my poetry course as well as my completely separate satire course. I did not disclose this to either professor. When I share this with people, half call the practice cheating, and the other half call it genius. My niece told me it would certainly be grounds for expulsion at her college. In my mind, I was adding a level of intellectual complexity to my studies. Was this an ethical practice, or was I cheating?

The all my universities made it quite clear that this is not allowed, but I've never understood why. It's your own work, so why does it matter if you've done the work a little earlier before the deadline than perhaps they anticipated? 

Some people in the comments thread suggested that it's because schools want you to do a certain amount of work to get your degree, but I don't think that's actually the case. You get your course credits, and by extension your degree, by demonstrating mastery of certain material or skills. They evaluate this mastery through projects and exams, but the amount of work you put in is irrelevant.  If you can knock off an A+ term paper in half an hour, you have clearly mastered the material and deserve your A+.  Conversely, if you do the standard amount of work - even if you do twice the standard amount of work - but still can't produce a paper that meets the standards for a passing mark, you haven't mastered the material and don't get to pass. If you can prove to both professors that you have mastered the material of their respective courses by turning in the same piece of work, the fact remains that you've mastered the material.

Other commenters suggested that a single paper could not possibly meet the needs of two assignments, and, before we even get into the question of ethics, would need to be rewritten from the other perspective to be suitable for the other course.  This may well be true, but that doesn't make it a question of academic ethics.  If a student chooses to submit a project that doesn't meet the project requirements as perfectly as perhaps it could, they'll get a lower mark.  Voilà, natural consequences.  No need to bring the code of ethics into it.  

The professor who taught my humanities gen. ed. course, an older, bearded, sweater-wearing gent who called male students by their surnames and female students "Miss Surname", had a policy that you can go to the washroom whenever you wanted during the exam, unescorted.  His reasoning was that if you can find answers in the washroom, more power to you.  His exams were designed so students have to analyze and to make cogent arguments supporting their point - things you can't put on a crib sheet.

Similarly, the attitude should be if you can reuse work, more power to you.  If schools want to discourage this, perhaps they need a more robust anti-requisite system, or more stringent academic standards, or a system that permits students to test out of courses where they've already mastered the material. But if you have two courses that are asking students to submit similar assignments to prove similar bodies of knowledge, then there's no reason not to permit them to do the same work.  And manipulating the academic code of ethics to ban this so they don't have to address flaws in the curriculum is kind of, well, inethical.