exactly like anyone else. They don't have special nominations. They are applying to the academy just like you. Athletics is just like any other activity on an application. And you aren't given an appointment to the academy to be an "Intercollegiate Athlete". You're given an appointment to be a cadet. Whether you play intercollegiate sports or not is something that will be determined once you get to the academy. On average, about 50 freshmen a year go to the academy with the hopes of playing football. They all practice and such. But by spring, about now; half of those freshmen will be off the team. They are still cadets, just not intercollegiate athletes any longer.
So, there isn't anything different between someone who is applying to the academy and wants to play IC sports and another applicant. There's a lot of people in both categories that don't get appointments.