Dogs have amazingly fast digestive systems, so bacteria and bad things don't have a chance to get into their systems, like with humans. That's why you often see dogs eating garbage, but never get sick. I've seen my dog eat a lump of canned dog food off the basement floor that had been there overnight-I hadn't noticed it the night before when I brought him upstairs-and he was fine!
Before we were married, DH's dog got into some Xmas presents we had bought, a box of four VERY rich chocolate truffles, and he was totally normal! I was livid..it was about $12 dollars worth of chocolate!