What's bizarre about it? You do the courses in school if you're able for the pace and standard of work. They're taught by the high school teachers. You register for the exams and take them in school. They're just one more exam, two in the case of economics, iirc.
My DCs took AP level courses from their second year of high school on.
Students are very often placed on different academic tracks from their first year onward based on the results of placement tests administered in their last year of middle school. Some students are going to test into calculus as freshmen, and some will be placed in remedial pre algebra. The majority will be placed somewhere in between. The same goes for subjects like English/ Language Arts - some kids will go on the honours track, and some will need to be taught basics like sentence construction.
In other words, high schools cater for a wide range of ability, and aspiration too. Schools provide courses as different as automotive tech / cosmetology and Latin / Individual Investigative Science Project. My local high school has 3,500+ students enrolled. They're not all university bound. They don't all need AP level classes.
High schools have to teach everyone the courses mandated by their individual state, and there are often individual school district requirements too. You can't graduate from the local high school without passing swimming and your driving exam, plus civics, financial literacy, art, a certain number of years of English, mathematics, science, mfl, applied arts, etc. It pushes some students to pass all the classes required to amass the 43 credits they need to graduate.
Generally, the most able students start on the honours track and end up taking pretty much all AP level courses in their final two years. Others stay on 'college prep' level, with others doing more vocational courses along with the required maths, English, etc. There is an opportunity to move up a level via summer school, where you can do a year's worth of maths (for example) in an intensive six week timeframe involving four hours of daily slog. Not everyone has the motivation or need to do this. Summer school also offers retakes of classes students may have failed.