Dogs, like children and adult humans differ in their temperament and their personality. Sometimes that's a breed issue and sometimes that's a personality issue. There are some dogs who are perfectly controlled right up until their prey drive kicks in, some dogs are more curious than others, some are food obsessed. It really depends. Dog owners, like parents are consistently risk assessing the environment (the good ones at least) and good dog owners are also risk assessing the dog.

If someone attempted to feed my dog, I'd react in much the same way as if someone drove on the pavement when I was walking my kid to school. I am responsible for the first part of the dangerous interaction (leaving the house) I am not responsible for the unpredictable behaviour of someone else. In this case, the cause and effect is the other person's behaviour has transgressed a norm and gets thumped. If I leave a kid unsupervised and they run in front of a car, if I let a dog harass the kangaroo - these are dangers I could've foreseen and mitigated against. In the case of someone presenting my dog with food - this is not a normal predictable thing to happen.

So the rules of cause and effect apply here, when I let a dog off the leash it's because I have done a risk assessment. I have considered the environment to pose normal levels of danger for a dog and am confident re-call will work except in extenuating and highly unpredictable situations. If those occur, then I take responsibility. In this case, the responsible action I've determined is to punch someone in the face if they attempt to feed my dog. Allowing people to go around poisoning dogs is the same logic as blaming people for getting mugged. It's your risk, you left the house. Shit happens.

Also.... as a transatlantic aside. We don't really have dog parks here in the UK. They aren't really a thing - we just have parks and dogs walk there all the time. Some on the lead, most off. It's quite the social event (for dogs and humans alike)