They know the publisher of the game who is getting revenue from it.
The know the thousands/tens of thousands/millions of accounts which has processed a purchase (probably hundreds of purchases per user) through it.
Some fraction of the user base is 'legit' which is to say they are playing for their own reasons.
I think the only way you prosecute that is that you find someone who was told, take this money, buy an iTunes/Play /Amazon gift card, use that card to buy items in this specific game. and finding that person would be very difficult. The dealers could literally hand gift cards to addicts for their kids to play their game while they are high.
I agree that there are pretty 'classic' schemes that could process more money but some of these online games are pretty amazing at the numbers they pull in monthly. Hard to explain a nail salon doing 5 million dollars a month in revenue.