She didn't' say that it wasn't rude; she said that it wasn't *wrong*.
Having lost the amount of livestock I have to people's 'sweet little pups that wouldn't hurt a fly', just laughing at it for getting chased by the goose is about the nicest and most polite response it's likely to get.
Misfortune is getting struck by lightning, and laughing is cruel. Sticking your finger in an electric socket after arguing with the person telling you not to is something else entirely, and the laughter at your expense is earned.
Letting your dog out of the car on private property, pointing at the chickens, and saying 'get up', is far more akin to the electric socket than the random lightning strike. It's not 'cute' or 'funny' even when the chickens outmass the dog, because they still run and can still get hurt due to panicking.
Yes. It's rude to laugh at other people's 'misfortune', which is why it is extremely rude to take your predator to another person's private property and encourage it to terrorize their animals, which may be livestock or 'loved pets', both of which I have lost to this kind of 'joke'.
Good etiquette is not letting your animal onto someone else's property without their express permission and keeping it under control at all times.