Poisoning the dog neutralises the threat without being seen to take direct action in front of an owner who is demonstrably irresponsible.

If the dog routinely chases people and it's likely only a matter of time before it causes someone serious injury or worse and the owner does nothing it seems to resolve the situation without the use of an obviously deadly weapon which would just cause more aggravation down the line.

I don't care about revenge on dogs, I do care about making sure a dog that attempts to bite me doesn't get any more chances than I can get away with.