Stuff in a dumpster / at the curb is still owned by the person that put it there until the trash company dumps it into the truck and mixes it with the other trash. This should also apply to oil.

The restaurant may or may not pay to have it taken away, I don't know the details.

I would think that the restaurant would need to account for oil entering and leaving the site to prove they are not dumping it down the drain / somewhere else. If the waste company comes up short, the restaurant might have a problem.