Probably fresh out of MP training:
I don't know the Army's regulations on when and where they're allowed to wear certain uniforms, but even if this guy was allowed to wear that uniform at that that time and place, he was embarassingly out of line. (Certain uniforms are forbidden unless you're in an "on-duty" status - though acting like bootcamp Johnny on a power trip is always forbidden.)
Unless his unit is full of like-minded soldiers, (which I doubt) he's going to be mighty embarassed when he gets back to base to answer for this.
He's the kind of MP who loves to pull fellow soldiers over and hopes they're a higher rank than him so he can throw down the ole, "don't confuse your rank with my authority."