The problem with allowing the students to take all the courses they want is that some of them can't correctly measure how much work they can handle. So you will get horror stories like: "A student get enrolled in 3 times the expected courses, has too many homework, too many midterms the same week, and finally don't approve any of them."
Now we have a lost of students that come with questions like "I didn't approve Calculus I last semester. Can I enroll in Algebra I, Calculus I and Calculus II, so I don't lose a semester?" And it is hard to tell them, but most of the times the best thing is to take only "Algebra I" and approve that than taking the three classes and approving none of them.
I also understand that there are special cases, but should be evaluated case by case.