He never impersonated an officer, dumbass. He said, "I'm sergaent Tom Mabe. We got a couple undercover agents...." and left them to assume the rest. To impersonate an officer, they'd actually need a fake badge or ID that is made to look like the real one.
It's no more illegal than an undercover officer suggesting that he is NOT police. In fact, you'll find that the amount of entrapment that police commit these days is disgustingly overwhelming, and most officers deserve life sentences or the death penalty for their crimes. That is the only sensible punishment when someone takes advantage of their position of authority in order to destroy the rights and freedoms of innocent Americans - it's analogous to raping a woman because you're a strong man, except that the rapist at least acknowledges that he is fucking someone. The police fuck society and don't bat an eye.
He CLEARLY intended to imply that both he and the homeless people were police officers (or some other law enforcement agency). That is impersonating an officer. Either way, he lied, and I'd be much more impressed if instead of tricking other people into spending their money, that he just grew up and spent his own.
Also, do your research first. Identifying yourself as a police officer or strongly implying that you are constitutes impersonation.
"Under federal law, pretending to be "an officer or employee acting under the authority of the United States" in order to demand or obtain "any money, paper, document, or thing of value" can result in a fine as well as imprisonment for up to three years (18 U.S.C.A. § 912)" - http://legal-dictionary.thefreedictionary.com/impersonation