Obviously; no school is going to give credit for simply taking an exam.
Your post made it sound as if the only way a student could hope to pass an advanced-credit test was if he or she had taken some sort of special classes to prep for it. That just isn't so. Sure, those classes would help, but if you have enough knowledge to score well it doesn't matter where you gained it. The CLEP credit path is also often used by adult returning students who gained the knowledge in their working lives.
There are an awful lot of people out there for whom the gamble of spending three or four hundred dollars to take a shot at several CLEP tests is very worth it, weighed against even the possibility of saving tens of thousands of dollars in tuition. For some it can mean the difference of getting to choose a more expensive school that would otherwise be out of reach financially.
Yes they do--the ONLY credit given for AP is getting a 3 or higher on the test-it has NOTHING to do with taking the class at all. Now, most highly competitive schools you have to get a 4 or 5 on the AP test-some don't give credit at all any more too.