Why do American universities require transcripts from every college?
Well they cant really take your word that you only took this or that class. Maybe you have a flunked semester you are withholding. And maybe you have academic dishonesty suspensions or other suspensions for aggressive behavior toward other students. They would want to know. All college classes are part of the permanent record no matter your reason for taking it. The college may use the National Student Clearing house to verify that you report previous college coursework.