In the Pacific Northwest, everywhere I suspect, we all like our pets. Some of our pets are larger than others. And many of us decide that we want our pets to be able to enter and exit the house at will. When a pet is large, the size of the pet door required to provide access can be a significant security risk. I remember at one house I inspected, when an agent locked the key inside, I was able to get back in by squeezing through the pet door. Since I weighed 200 pounds at the time, that was not a small nor a secure pet door.
This pet door (above) is a good example of an entry point into the house that an uninvited human could pass through. Not only could an uninvited human enter, so could rats, various other rodents and even large animals you do not want in the house such as raccoons. I have a neighbor with a pet door and he has to clear raccoons off the back porch several nights a week and that species can be nasty.
Holes such as this pet door compromise the integrity of the house door and they defeat any efforts at energy efficiency and weatherproofing. Often, buyers plan to replace exterior doors that have been damaged by pet doors. Other times, the buyers have big dogs themselves so they keep the altered doors.
Large pet doors may accommodate the big dog but, if the big dog is on an outing, or if he moved out of the house, then that big pet door is a great big open invitation to a burglar.