Because warnings are a GOOD thing, that’s why. If your dog is growling, it has already tried to communicate with other non-aggressive body language that it is not comfortable in that situation; but that communication failed 
him. Now he has had to escalate his behavior to have his voice heard. If you punish your dog for growling, you are doing nothing to alleviate his anxiety BUT you ARE taking away his ability to tell you…please DO NOT make me bite you to get you to back off.
You can not stop a bite once it is in progress. BUT you can absolutely prevent a dog bite, while also building positive associations between your dog and whatever or whoever it is that is causing him to be anxious.
