I've gone dumpster diving on a couple of occasions with some friends of mine. I actually think most people should, at the very least, watch other people dumpster dive at least once. You will be absolutely amazed at the amount of perfectly good food that gets thrown out.
Go to a bakery right after they close. There will be garbage bags full of bread (and nothing but bread so it's relatively sanitary) that was baked that day. Even if you don't eat any, it will probably change your perception of waste.
When I was a poor starving student and worked in a supermarket the managers had to put all the expired cakes and cookies in the trash compactor to stop workers taking them. Not sure if this was so we would buy from the store instead or in case we sued if we got ill
I believe the reason is actually to avoid intentional over production. If the employees know that they'll be able to take home any left overs, then why not bake a few extra cakes... that kind of stuff.
I'm still a student but when I was much poorer and starving I raided a Dunkin Doughnuts dumpster and ended up with an industrial sized trashbag of every doughnut they made sitting in my living room for 3 days, until me and my roommates got sick of eating nothing but doughnuts and threw them out proper.
Go to a bakery right after they close. There will be garbage bags full of bread (and nothing but bread so it's relatively sanitary) that was baked that day. Even if you don't eat any, it will probably change your perception of waste.