I think it was the father's own fault. As the article said, these places are mostly staffed by teenagers. They get distracted, they make mistakes, some of them probably don't even understand what a peanut allergy entails. The father should have double checked the drink, or even better, should not have taken the kid in there for a drink in the first place.
I have had several friends who had severe nut allergies, or their kids did. Some of them can't even bring the kid into a room where nuts or peanut butter were recently used, as there are particles in the air. Was a smoothie really worth such a risk?