A cadet does not NEED a tacvest. A cadet does not NEED webbing.
I can show you a million people who do strenuos activites all over the planet that do need use military style webbing to transport their water.
That being said, cadets (in particular Army cadets) love Army stuff. The more "real" it is, the better. It is ALL about the "look cool factor".
There is nothing wrong with that and when I was a cadet I saved up to buy myself an 82 pattern webbing. I then joined the reserves and had a set issued to me, and then again in the regs.