The few locals trying this has turned into hoards on high travel days, and the smooth talking has become outright harassment. Tourists are left with this as their final New York City memory, being grilled in a language they don't speak, in an elevator that smells faintly of urine. They open their wallets and hand over the cards to appease the person, but it's a tense situation. The scammers then go off to verify the value or days left on the cards, re-sell them on the street, and put that cash into other illicit activities.
We first saw this happening with a few scammers hanging out in the AirTrain station, before the turnstiles. That was a little over one year ago. On this most recent trip out of JFK last week, we not only witnessed tourists being harassed in the elevators and escalators, but even down on the subway platforms. Typically we are left alone because the scammers target young foreigners lugging major baggage (we put on our New York scowl and travel carry-on only). That doesn't mean we're blind, however.
We have noticed an increase in the police presence at Jamaica Station, but it's so hard to catch the Metrocard scammers red-handed.
How to spot (and avoid) a Metrocard scammer: They're alone with no rolling luggage, but stick close to groups of travelers hauling luggage. They wear clean, new-looking casual clothing but they themselves look pretty run-down, like seasoned pickpockets on a good day. They bother only a few travelers and then quickly exit the scene after scoring some cards (they're circling back around to start again).
As far as we can tell, there's been nothing written about this and YET it's an issue snowballing into dangerous territory.
As always, be safe out there and don't talk to strangers.