At our local park, dogs can play off leash as long as owners have “voice control” over the dog. There are at least two reasons a dog might come when called at the dog park. One reason is that it’s being forced with loud demands and shrieking whistles implying a future punishment should the dog refuse. The other reason is that the dog has an incentive to come to your call. For example, if every time you call your dog, he gets a treat or a back scratch, he will learn that he has an incentive to respond to your call. It’s the difference between power and force and just another example of basic economic realities that are present everywhere, even at the park.