What happened to the idea of 'promoting the progress of human knowledge'? Copyright certainly protects the author's interest, but how could you teach using the works of others if copyright preserves the five exclusive rights solely for the copyright holder?
You would have to create all learning materials yourself as you could not take advantage of the work of any of your colleagues in teaching or in the advancement of science and knowledge. That would be crippling to the stated goals of advancing science and devastating for educators!
Indeed, that is not the intent of copyright.
The Copyright Schematic below may help you understand how copyright and copyright exemptions for educators are related. Don't study it too hard right now because we will look at the individual pieces. Notice there are three highlighted boxes.