well - to play devil's advocate, it's clever but if a student hoarded books, he'd have the book thrown at him/her and academic standards should apply to all equally.
Well students are expected to come to the university, study in obscurity, then leave after paying monies.
The context is different for professors, especially tenured ones.
Why do you think this is an "academic standard"? And even if it is one, why should they apply equally (e.g. its an academic standard that students cannot use one paper for multiple classes, however professors can use one paper for multiple conferences/journals/etc.)
To answer your last question, because the purposes are different.
Professors assign homework (including papers) to force students to learn something new. But professors speak at conferences because other professors want to hear about their work.
It varies a lot by discipline. Law reviews universally accept papers that have been submitted elsewhere; CS conferences universally don't (unless they were rejected already, of course). Lots of places, in many disciplines, will publish material in technical reports or dissertations that's already been published elsewhere.
