The greens hate the shale oil, they look at it as just perpetuating the use of carbon based fuels and the massive strip mining to dig up the shale. They love this grease energy but even if all the grease in the US was turned into fuel would it even put a dent in demand for gasoline? I can imagine this grease thing taking off and people lining up at McDonalds for grease and McDonalds charging for it and then all of a sudden its not so cheap to run your vehicle on grease anymore.
The anecdotal stories here talk about driving to local eateries to get the free grease with no mention of cost for the time or fuel expended to do this. And yes, you are correct, there ain't enough of it anyway.