Food returned to stores is often tossed out, I know they do at Super Target and my local grocery store.
I live next door to a bread thrift store, they set aside some expired bread to sell for bear bait/animal feed. The remaining expired is put in a LOCKED trash dumpster to prohibit trash picking. I find it sad to see the clerks toss out grocery cart loads full of bread several times a week.
I know Super Target has barrels full of spotted bananas in "back of house" storage areas, they have strict guidelines on produce appearance. I'm not sure if they toss them or give it to the pig farmers (the scraps from the cut produce is set aside for farmers).
Reclaiming produce, bread and other short shelf life items is becoming more common for food shelf/soup kitchen donations.. but it takes manpower, planning and trucks to ship it where it will be used.