Because they are not being abused or ill-treated. They are given special training, well-fed and looked after. I don't see how this is any different to any other kind of dog training (eg behavioural training)
I agree with 5029. Animal welfare is dear to me heart, but in TV adverts there are very strict guidelines to ensure that it is not compromised. What I do dislike are adverts where animals are dressed up, as I find these demeaning to the animals' dignity, but again it's not cruel and anyway it's an anthropomorphic reaction.
Other answers when you posted this again included: 'Because animals (especially young ones) are appealing to humans. When a consumer feels 'fuzzy', they are more likely to view the advertised product in a positive frame of mind.' (this from wildwood) and 'I think corporate types call it "The Ahhhh factor".' (Cactus)