Well, you might want to cross-check the list of high schools offering classes beyond calculus with those that have multiple kids making USA(J)MO before writing off kids that take those classes as having gone through a bunch of fluff courses. IIRC, there are a couple high schools that have more than one kid at that level - I doubt that those kids are weak in problem-solving and proofs! I think it is quite possible that both of the following are true concurrently: (a)there are many kids getting through HS math courses that are weak and (b) there are schools that have kids who need college level math prior to graduation from HS.
To answer the question of why one would take math after finishing Calc BC, should one do so as a freshman/soph - because a person taking that path probably wants to go into a field that uses math and will be taking further math courses in college. It is not a good idea to just take a couple years off math and then try to jump back in, even if such a person would want to do so. Pretty unlikely anyone would want to anyway.
As far as dealing with HS graduation requirements, perhaps it is difficult with virtual high schools, but every b&m high school I've talked to has an option in place to verify geometry through testing if the course isn't already on the transcript. It's really not a problem, and all other math (as well as foreign language) courses follow the rule that mastery of a course is sufficient to verify mastery of courses that are prior to it in the sequence.
Ditto to everything kcab said -
polarbear
