This is an organized crime thing, apparently there's a chinese mob?

These people were stealing oil from restaurants and selling it to downstream users for industrial uses (making biodiesel is one)

Google suggests at about $0.5 per gallon

I'm not sure what the restaurant gets paid for it, probably not a lot, they may even have to pay for the service like they do for trash dumpsters. But unlike trash, the oil has a value so they probably do get paid a little bit.

They are also legally required to dispose of waste cooking oil properly. It's not toxic per se, but you can't just dump it down the drain.