And you probably aren't expected to spend your free time grading papers, writing exams and planning classes.
Seriously, if I assign a class 5 minutes of home work to do, that's a minimum of 20 minutes per class that I have to spend grading. And that's just for a very quick look and check mark to grade. There's easily another half hour planning the assignment so that I get what I'm asking for rather than something that's technically in compliance with the instructions.
Teachers spend a lot more time working than what is represented in the class schedule and thanks to budget cuts, end up spending a lot of time doing things like copying and research which really ought to be done by support staff so we can focus on the students.