Are you kidding? This is who you are? It's private property, chief. Frolic in mine and I'll show you precedent.
I am someone who, when told that something is 'illegal' which may or may not actually be illegal, asks for proof. The legality of dumpster diving is definitely not that cut and dry.http://freegan.info/what-is-a-freegan/freegan-practices/urban-foraging/diving-and-the-law/
Most dumpsters are located in parking lots. If someone parks their car in a restaurant parking lot, are they trespassing? Of course not. I'm in no way saying that dumpster diving is legal, and, on more than one occasion, I've warned people not to dive, or to be hyper careful, because neanderthal restaurant owners (present company included) can be dangerous. I live in an area where people have been 'disappeared' over restaurant recipe leaks, so I take this kind of thing seriously.