Workplace descrimination?

Recently, me and two of my coworkers were bored at work so we decided to go across the street and fly a kite on our break, management found out about it, and one coworker and I got a write-up for going off campus while on the clock. I understand why we are being punished, but the thing is, the third person was not punished at all. We are all males, the same age, and all caucasian, so gernder/age/race descrimination wouldnt apply, but i was wondering if anyone had any idea if this is legal or not? I'm going into HR to talk to them about it tomorrow but i would like some further insight into the matter first. Thanks