Depends on the university or college offering the class. Open enrollment courses without a paid subscription does not always equivocate college credit for the course. Some colleges provide courses to anyone interested as a tutorial service. For example, at my college, a math instructor has posted a college algebra course that is open and free to anyone who wants to download it; but it cannot be taken for college credit unless you enroll at that college.
We have high school students who dual-enroll; meaning they take college courses while finishing high school.
SO, if you find a class you would like to take in iTunesU for college credit, follow up with that college/university to find out if you can receive credit.