My school changed it so that the AP US History test only counted if you got a 5. If you got a 4, you were exempt from taking US History, but you didn't get credit, so you had to take another history class instead.
If you made a 4 on the AP US history exam, you most likely learned more than the average student who made an A in college US History.
They also severely limit credit by examination. It's all about making people pay for classes.