On the math not being guaranteed: most math departments require a placement test to place out of classes. This isn't a nefarious plot; it's for the kid's own good to be placed in a class where he will succeed. Math is easily measured; English/Lit proficiency a bit harder.
I'd argue that APs have a bigger problem in being able to place out of classes. A HS class, taught by HS teachers, to HS kids, with HS study habits, and being structured to teach to the test is an illusion of a college level class. Between my two kids there were 16 AP classes. Not a single STEM related AP class covered college level material adequately. Classes at UCSD did - and of course were far harder than any AP class - primarily because tests in college are way different than HS regurgitation type tests.
My son (athlete) described his AP classes as covering the low hanging fruit and leaving huge holes in conceptual understanding when compared to the "same" college class. The problems compounded if you used that AP 5 to skip the intro class.
Many kids (at their HA) took the HS APs because thats the "rule" to check the most rigorous curriculum box, but took the same class once they got to college. They still had to work hard to pass. Additionally, as my oldest learned, a freshman taking an upper level class is at a huge disadvantage in mastery of the prerequisite material AND college study requirements and college tests.
IMO, believing most HS kids have acquired college level learning by virtue of AP classes is a delusion created by the admissions race. While a kid is forced into APs, don't believe he's mastered any college level material - regardless of the ultimate AP score.