Coprophagia (eating fecal matter) is fairly common in dogs. There are varying theories on why dogs do it (maternal training, zinc deficiency, etc). It doesn’t tend to create problems but in some cases can cause diarrhea from a bacterial imbalance (and it certainly increases the chance of injesting intestinal parasites). For those with a pet with this quirk, there is a product called “Forbid” (it’s actually meat tenderizer) which, when sprinkled on the animals’ food, causes the poop to be unpalatable (hard to imagine that it was palatable in the first place). Of course, the product only works if they are eating their own poop.
As to why it doesn’t make them sick, I’m afraid there seems to be very little research and only anecdotal answers. In my own opinion, the fact that dogs are ominvores and opportunistic feeders would seem to require that they have a more sturdy digestive constitution.
My (even more out there) personal opinion is that we as humans have weakened ourselves through the use of durgs and antibiotics. If you read the history of modern medicine, you will find that (some) people would voluntarily hold still while having ivasive surgery such as gall bladder removal and appendix surgery without the aid of any anesthesia or pain control (because it hadn’t been invented yet). During the civil war, soldiers would have limbs amputated with rusty, bloody, fly covered instruments. In all of these examples, the surprising thing isn’t that many died, but that some survived.
I see dogs walk into the clinic with broken legs swinging freely and wagging their tails. I see cats that have fallen from windows and been pierced belly to back by a broken branch and survive. I have seen animals survive things that I know a human most likely would not. I think that they don’t have as many repercussions for doing something as questionable as eating their own fecal matter because they’re tougher than us.
Ok, done with my rant now….sorry.