What, the dealer also holds a gun to your head? If you can’t afford the payments, don’t buy the car (or the house, etc.).
Its not a clear cut scam, honestly. There are people who abuse the system (in the same way people use and abuse in house credit card sign ups in stores like best buy or circuit city), but it’s also a genuine business model.
Buy an inexpensive car, and sell it with enough mark up to cover the likely loss and termination of the contract (which happens at buy here pay here often, there’s a reason people have shitty credit – and its not usually because they pay their bills on time), possible distraction of property, likely litigation fees, etc and hope for the best.
Buy here pay here lots make more money on people who pay than people who don’t, at least decent ones (in my experience).
You are correct that some people abuse the system, but its not a scam – the terms are, or should, honestly be laid out up front.