An Ethical Question, sort of.

In the midst of writing the second of my five massive papers to finish my career as a master's student, I thought of an issue which has come up quite frequently in my courses lately: using your own work from another course. Example: I have two classes on similar subjects and write one paper and turn it in to both teachers. Apparently, this is a big NO at Harvard. Don't worry, I don't think any of my papers have any overlap, so this isn't even a possibility for me, even if I wanted it to be. But here is my question - I asked Clarke and he couldn't think of a good explanation - and we would like to solicit a Morgan answer (and anyone else who feels the urge to comment). We will take both the Morgan answer as 1) a lawyer and 2) a former student who hated writing papers as much as the next kid, and entertained, or maybe did, use his grandma's old book report. Both answers would be nice. I am wondering why this is such a big deal. It is my intellectual property - I put the blood, sweat, tears and brilliance into it and I don't think it's cheating. I can understand handing in my somebody else's paper as a bad thing, but my own?? General authorities do it all the time - they use old talks or devotionals in the Ensign, businessmen will give the same lecture to multiple conferences and I know I've heard teachers give the same lectures to different classes, so why can't my paper cut it as legitimate work for different teachers? PS - the picture is to grab Morgan's attention.