(You mean they don't?)
Dog owners trust a variety of "professionals" to care for their dogs - veterinarians, vet techs, groomers, kennel workers, pet sitters, dog walkers, shelter/rescue workers, pet store clerks, trainers and behaviorists. Most dog owners - I know I made this assumption - think the people caring for their dogs are well trained and will give their dogs the same quality of care they receive at home.
In reality, the only animal care professionals who are required to be licensed are veterinarians, vet techs and nurses. Other dog-care workers may belong to an association - or not, may have received formal training - or not, may have received GOOD training - or not, may stay abreast of current developments in their field - or not.
This is not to say that certification or formal education guarantees that someone is qualified to work with dogs, as the case of Spork has shown us. It certainly doesn't mean mistakes won't happen - even the most qualified and conscientious professionals don't always do everything perfectly. And of course, accidents do happen.
But what dog owners - all pet owners - need to be aware of is the number of under-qualified, disinterested and even unscrupulous people who may be "caring" for their dogs. Even in professions where you would assume there would be some level of formal education - grooming or training for example - there are many self-taught, over-confident and under-qualified individuals passing themselves off as knowledgeable professionals.
And despite their claims of success and promises to fix or rehabilitate your dog, they may not really have the knowledge, the empathy or the personality to do what they claim they can do.
So how do you find someone you can trust? Find someone who belongs to an respected association. Get references. Check those references. Ask about experience and education. Ask about their training philosophy. Ask about how certain situations are handled. Make spontaneous visits to check on how your dog is being handled.
And if you suspect your dog has been a victim of poor handling, for example if they suffer from a drastic personality change after a visit to a kennel, groomer or trainer, don't assume the problem was caused there, BUT, do ask some pointed and direct questions.