All the answers on why dogs eat cat poop concern a variation on ‘normal’ behavior. I’d like to suggest that it is abnormal behavior as wild dogs don’t do it and the reason could be psychological.
I’m going to answer the question in a slightly unconventional way. The conventional way is to simply state that cat poop contains some nutrients and in eating it the nutrients are being recycled (re-digested). Another conventional argument is that dogs who eat cat poop have an enzyme deficiency due to a poor diet such as being fed on entirely processed dry foods which drives the dog to look for sources of digestive enzymes. Or it’s a way of dealing with intestinal parasites. These are scientific reasons associated with nutrition.
The interesting aspect of this question is that if you search in Google for “wild dogs eat cat poop” you are invariably presented with tens of thousands of websites discussing why domestic dogs eat cat poop. There’s nothing on the Internet about wild dogs such as wolves or African wild dogs eating cat poop or any other poop for that matter. The reason why I searched for information about wild dogs is because I wanted to see whether this particular apparent aberration is unique to domestic dogs within the canine family of species and I think it is.
Other animals apparently do eat faeces as normal behavior such as rabbits and hares but as far as I can gather wild dogs do not eat poop and if that is the case then it seems that there’s something wrong with the way that people are looking after domestic dogs. There’s something wrong with the domestication of dogs. This takes me back to the opening paragraph about a dog’s diet being deficient. Perhaps dogs are looking for something which their provided diet cannot satisfy. Perhaps the nutrient they are seeking is in cat food and remains partially undigested and therefore passes out as faeces. The fact that some dogs eats faeces in the cat litter is unimportant I would argue. It just means that the faeces is in the litter and therefore a dog would eat it from the litter tray.
There may be another aspect of the life of a domestic dog which results in this aberration. Perhaps their lives are boring, too solitary or not natural enough. Might it be an expression of frustration, anxiety or some other emotional distress? Sometimes humans eat faeces but it has been observed in people with schizophrenia. Cats sometimes eat wool and other indigestible non-nutritional items. Both are abnormalities in behaviour. All the information more or less explains dogs eating cat poop as ‘normal’ in some way. None have discussed the possibility of psychological distress. It needs to be explored more. There may be something fundamentally wrong.
The two obvious reasons why dogs eat cat poop out of the litter box in my opinion are (a) dogs are looking for a nutrient which is not present in their diet or (b) it is an expression of a slightly disturbed mind due the unnaturalness of their environment. If it is the former why is it happening day in and day out in the West? Surely the pet food manufacturers can fix the problem. This leads me to believe that the reason is the latter, something far harder to resolve.