Tattoo for your pet

Recently I read an article from Dogster about a tattoo artist.  The tattoo artist made headlines because they managed to give their dog a tattoo while he was under.  The tattoo artist eventually lost his job and there was outrage across the US.  What is really upsetting is that no one mentions that the Veterinarian actually gave the owner permission to conduct this procedure while the dog was under.  This is completely disgusting to me.  Why would you tattoo your pet?  What is really surprising is that if you google Tattooed pets, you will see articles and images of cases such as these.  Is this not animal cruelty or is it just me?