Second, if a thief comes onto your private property and takes what you own it's stealing. Used grease is not placed in a 'dumpster' it is either held in a in ground storage tank or in barrels locked in enclosures called a grease safe. Either way a 'thief' trespasses on private property and breaks into a locked receptacle to take what does not belong to them. That's how it is stealing.
I guess you would be OK with someone going onto your property at night and taking some of your property that they figured was trash.
We live in a condo and each courtyard has a trash bin. There are people that go through the bins looking for things they can sell or recycle. I don’t have a problem with that.
The way the headline was written made it sound like it was “in the trash”, rather than in a storage container to be sold.