Students who knowingly give classmates higher grades than they deserve while we grade papers in class will lose points on their own assignments. If we cannot grade each other's homework in class then I will be forced to collect them to be graded personally, and students will lose that time to review their work.
I realize that many students do not like having their homework graded and I understand, completely, the pressure to overlook some wrong answers when it's a friend's paper. This is not, however, an excuse. Homework is not "just" practice, and we cannot wait until the day of the test to worry about knowing how to do these problems.