IEven in a situation where AP/IB credits are a poor approximation for freshman classes, the end product of a degree program at an elite university is a student who can produce high caliber fourth year work. That student will have to contend with upper class material upon matriculation, and be competitive within that cohort. As long as the grading mechanisms within the institution are internally consistent (big assumption, I know!), students who are under-prepared will not perform as well in later coursework.
If the university determines that the AP or IB classes don't meet their standards, why would it want to set students up for problems in upper division courses?
If I had gone into second-year English or History classes based entirely on my AP classes (I got As), I would have crashed and burned, and maybe ended up repeating a year or going to a less rigorous college.
People here complain that poor HS preparation leads to remedial math and English courses for many students, and contributes to poor outcomes. Why should we allow poor preparation a step or two up from remedial? Why pretend AP classes accurately reflect what should be expected from a first-year history or English class at a college with high standards? Remember that AP classes were designed to mirror community college classes, and CC classes are generally easier that classes at more rigorous colleges.
It may be that you're focusing more on the total cohort's performance, given that you have a post-secondary educator's lens, whereas I'm looking at how the policy affects students like myself. I see this issue as analogous to grade skips in K-12; if the student can do it, let him/her; else, let the student incur the natural consequences (repeating the class, moving down a level).
I grant you that simply promoting students into the next level without the requisite skill set is undesirable.
This also speaks to the need for communication within departments with potential accelerants. When I matriculated, I met with the undergraduate program dean to assess my suitability for taking on upper-year coursework in my major, and for taking an additional course in my courseload. The rest of the credits granted were based on measurable performance (languages, calculus). For one language, I met with the department head and received placement into an upper level class. For calculus, I could have simply taken the end-of-year exam as proof, with or without IB/AP placements.
To synthesize what is otherwise a horribly rambling post, I'll say this: students should be able to contest the final evaluations of whatever courses they like, whenever they like, AP/IB credits or not. If you can handle the material and thrive, do it! If not, follow the wisdom of the deans and map your course plan accordingly.