I am a hobbyist writer and have been researching K-9s. I've found several articles saying that many police officers (even SWAT officers) are afraid of K-9s. It seems that many of the dogs, while perhaps good at some aspects of their work, are not friendly or particularly well-behaved in other ways. It also seems to be the case that high worldwide demand for K-9s has made finding good ones more difficult and expensive.
While I respect and like working dogs, I sure as heck would never pet one unless given permission!
My 10-year old is still scandalized from when she saw some little kid go up and hug a sniffer dog at the airport. On the other hand, I think it was the Denver airport that had a very cool Wolfhound that you were allowed to pet.