I think there are a lot of reasons. In my hunt for a dog, I talked to a few people about this. Dogs that breeders pair up with the specific intent of producing working dogs could have a propensity for producing "hard" dogs, a possibility of dog aggression, dogs that may be prone to handler aggression in response to unfair (and/or inexperienced) corrections, dogs that are so high drive/intensity that even a very active pet home would not be suitable, etc.
In my hunt for my current pup (I was looking at GSD and Malinois litters, and a dog for IPO), I almost never found a breeder labeling a litter "working home only" just to deter pet owners- if they said it, they really meant it. But, those breeders spend enough time getting to know potential buyers that they were straightforward over what dogs would and wouldn't be an appropriate match.
Last edited by LoveEcho; 03-28-2014 at 04:30 PM.