Because the recycling symbol is just wrong. I am not recycling the book/format, I am deleting it. So I replaced it with a trash can icon from some thread or another.
It may be called the recycling bin on Windows, but on Ubuntu it is called Trash, and thus on Ubuntu I have changed the icon. As in most things, Windows is wrong here, and it sounds stupid. Hey, let me recycle my files! Well, where am I reusing them?
Recycling is for materials that can be repurposed for new stuff, a paradigm that does not fit here.
However, I may heed your advice and not change the icon on my Windows install (when I get back to it, it's at home and right now I'm in school.) Clearly it fits in there with all the other Windows "quirks". I hadn't thought of that, to be honest.
Live and learn, indeed!!