You dont train a dog to come back when you call it by just letting it off the lead in a park and calling it until it comes back. Jesus I did not expect people to actually think it's acceptable for dogs to go up to strangers and not come back when called. I was just having moan 
You train a dog to recall by using extendable or longline leads and by practising in enclosed private spaces.
Let's say I didn't shout to get his dog three times and make sure he had it in his car before setting off. I could very easily have ran it over as could any of the other cars in the carpark. Or my dog could have bitten it for getting in our car.
It didn't hurt us but it did set her training back and now she'll be anxious at the park for at least a week. It crossed the boundary of her safe space, it will be a massive job to undo that damage.
Your dog should recall reliably under usual conditions, if people, food, dogs, balls are likely to prevent your dog returning to you when called it needs to be on a lead as it doesn't have recall.
My dog does not recall. We play ball, she explores the woods she plays with friends. Always on a lead or in a secure environment.
Also, I could very easily have stolen the dog, I could have been on the main road out of sight by the time he'd realised his dog had gone.
Controlling your dog is protecting your dog.