A couple of things that I thought of later:
• I think if you check the admissions agreement, they also use language like “expect you to maintain your GPA” or something to that effect. Just because they “expect” this does not mean that they always get it. Similarly, despite the wording, the student with all A’s who suddenly gets a B, will not be rescinded, although clearly, the GPA dropped. So the person may have simply been reading a script
• I think Princeton’s comment would have more validity if they were MIT or UCB or some other school that does not use the Common App. If a student applies using the Common App, s/he’s listing her projected AP exams based upon covering the requirements of the majority of schools. For a college that does not give AP credit in a subject to expect a student to take said exam is a little unrealistic IMO. Although, as you will soon find, Princeton will get you coming and going with fees and extras, I’d be telling them to pay me the $92 to take the exam if they want it so badly, or at the very least, know that I will be deducting the $92 from my annual giving. 
Anyway, best of luck to your son.