Well I think it's up to the owner to research on there own the breed they are looking into and not rely on the rescue or breeder. Also yes, it may not be the handlers fault they became aggressive or reactive but it is the handlers fault if they don't fix it or hire someone who can. Dogs can be rehabilitated and trained, it's the obligation of the owner to do it or get someone who can.Yes but its not the owners fault the dog is aggressive or reactive most of the time. Sometimes people do end up with a dog or breed they where not expecting and don't have the knowledge or experience to know what to do or where to find good information. I am not saying there aren't a lot of people who do stupid things and should do better but just because a dog is reactive or aggressive does not mean the owner is a bad owner or trainer
There are unethical rescues that will lie about the dog and ones who will downplay the issues the dog have (99% of imported dog rescues) which is one way somebody can end up with a dog they can't handle, then they might go for help and end up getting bad advice and misinformation (which is extremely common).
Training a reactive dog also takes sometimes putting your dog in a position where things might happen so dog ends up reacting
With all of the said there are far to many people who ignore genetics and have no idea about dog behaviour or body language. I see people being stupid with there dogs regularly and they should be held responsible!
Also if you are rescuing a dog, you should spend a lot of time with it to see for yourself the dogs temperament and behavior, you shouldn't just trust what the shelter says.
Imo everything can be avoided for fixed, its not easy but it's possible and the responsible thing to do. It's not the dogs fault an owner can't/won't help the dog.