And they encourage you to take easier classes because it'll be easier for you to get an A which means teachers get paid more!
Who cares what the grade is if the students aren't learning anything? The point of the education system is for students to learn, and grades are supposed to be an accurate measure of that. However, grades don't mean much anymore, at least that was the case in my high school. There were students who maintained an A in AP Calculus because of the way the teacher graded. People could miss all but one or two questions on multiple choice tests and still receive a 79% on them. Then, the students could do test corrections to get 2/3 of the points they missed back; he told us the answers to the tests.
Basically, if you enrolled in AP Calculus, you were guaranteed an A. You weren't, however, guaranteed any knowledge from the class.