Really sorry to hear about you losing your girls
. Like you say there is a good chance the puppy could have broken free so lets hope it was that but it doesn't stop the fact that there are some thoughtless owners who just don't seem to care what their dogs get up to.
As both a dog owner (also a Staff who is really lovely & friendly dog though I must admit I wouldn't trust her with the hens) and hen owner I can really understand your frustrations.
Really glad though that this hasn't put you off keeping hens.