I have had many courses in which the answers to the homework are given prior to the homework assignment. The assignment is large enough that copying the answers would take about an hour. Most students feel that if they are going to spend that much time faking it, they might as well do the assignment. Furthermore, biweekly quizzes clearly highlight a lack of understanding. Not doing the homework (or just copying the answers) is a rough road to go down, when averages midterm scores are in the low 60s (and yet some kids ace the midterm). Those who copy the homework do badly in the course. It's just that simple.