Why do people do it?
Inevitably it's small dog owners.
They see another dog approaching, their dogs are also off lead. They quickly recall and pick them up even when the other dog shows no aggression.
It just encourages them to fear the other dogs.
I have a Staffie mix that likes to play with other dogs. He doesn't go up to on lead dogs and is very gentle.
There are a few in the park that would recall and pick them up when they see him.
Today I saw one of them so I decided to turn the table round.
I shouted to my dog to come back, picked him up (all 20kg of him) and ran while screaming killer dog as the little one started yapping.