Probably since you are not commissioned as an officer yet. Cadets have historically been allowed to select rank as an enlisted person, in order to get use of Online TTX software for free.
.....and if you get bumped out at the 2nd or 1st class stage (Jr and Sr) before graduating, you end up having to commit to serve in enlisted ranks to cover your required service time.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
