This is the world we live in. Words, phrases, and actions are, regardless of context or intent, only okay for some races to use.
There wouldn't be an investigation it white cadets using what could possibly be a politically motivated black power symbol. But, they wouldn't be an investigation into a group of black cadets calling another black cadet the n word either.
I'm generally not one to want everything to be PC, but when a group of military members could be construed as doing something possibly offensive, it should be looked into. They represent the military as a whole, regardless if they mean to or not, and represent our country to the outside world.
I certainly don't agree with these cadets receiving anything more than a reminder their actions reflect upon everyone though.