It's the owner's fault, no matter if they meant to do it or not - if you have a dog that likes to snatch food, you don't risk killing it by making/having hash brownies. Simple.
I know. The poor thing. Stuff like this just makes me so angry with people. Who leaves pot brownies around when you got pets that don't know what they're eating? :(
For anyone who has a dog and is terrified of them eating chocolate, here is a little tool to help you decide whether to rush them to the vet or not:
http://www.petmd.com/dog/chocolate-toxicity
http://www.petmd.com/dog/chocolate-toxicity