They get the money from the peer networks that are the source of the spammers. If they don't cough up then you void your peering agreement. If they try to route around through a different trusted peer then that peer takes the hit. If its your own customers then you can do something about it. VOIP peers would quickly run into trouble and they'd probably require new customers to keep a spam deposit, carefully monitor their calls, and have a ramp up period that limits their outbound call rate and lifts it over time.

I don't see a downside yet.