Yes, labels indicating the toxicity of Xylitol to dogs should be required on products containing this substance. While Xylitol is harmless to humans and fully suitable as an ingredient in consumer food products, this substance has been proven harmful to dogs. Governments should ensure that the public knows which product contain Xylitol.
Many people feed their dogs 'people food'. Peanut butter is one of the treats that people use to get their dogs to take medicine or to simple keep their dogs occupied. Accordingly, any 'people food', especially peanut butter that contains something toxic to dogs should be labelled accordingly to put dog owners on notice.
If the ingredient is not banned outright, then it should absolutely be mandatory that the peanut butter companies must clearly state on the label that the product contains ingredients that will be harmful to dogs if ingested. This is simply a good measure to try to mitigate any negative consequences that may result from the products use.
It isn't the food company's or government's job to make sure dog owners don't feed their dogs toxic substances. I am a dog owner, and I always read the label of every food I intend to feed my dog. I don't need a label telling me to do so, and neither should any other dog owner. If you can't take the time to educate yourself on what foods or additives are toxic to your dog, then you shouldn't own one.
People who have dogs should learn enough about them to know how to take care of them. They should know what their dog can eat and what they can't eat. There is no way that a person that makes a product can think of everyone and everything that they have to warn about the product.