"Here's sommat I don't get. If a dog kills an innocent person, why does the dog owner not spend time in prison for at least the crime of manslaughter?" gnu

In the US, gnu, it most certainly does happen. Not long ago the owner of two dogs that killed a woman in San Francisco was first convicted of murder on the basis that she knew her dogs might kill. On appeal the charge was reduced to manslaughter; the defense argued that she could have known they might cause injury but that she had no way of knowing they would kill. She served more than two years.