Owners who let their dogs off leash in on leash areas have been rightly lambasted on various websites.
However, my other "pet hate" are owners who have their dog ON LEASH in off leash parks. The logic of this has always escaped me. Innumerable parks allow dogs on leash. Only a few allow off leash dogs. Are the leashed dogs meant to have fun watching other dogs run and play? Most dogs feel pretty unhappy being approached by an off leash dog if they themselves don't enjoy the same freedom. Some will cower, some will lunge. They will definitely be uptight. Worst of all, many them also get reprimanded (or even punished via the choker chain) by the owner, for straining on the lead. A perfect setup for developing aggression. Which was perfected by an owner in my local off leash park today: Not only was the young GSD on a leash, it was tethered to a goal post, with the owner sitting some way away, watching a Rugby game on the other side of the park. Just keep your dog away from leashed dogs then, some might say. Now, I have a pretty decent recall with both my dogs. So yes, I can keep them away from a dog like this: but that means I have to work pretty hard. After all, a new dog in their local haunt intrigues my two socially happy dogs. Why can't we go and say hello? When I go to the local park, I want my dogs to have a good time. I do a lot of training, but at the off leash park, we relax. So I'm not keen having to keep them firmly in check, it's not my idea of relaxing.
And of course accidents happen: As I was focusing on Giro, trying to steer him around this tethered dog (by now I had leashed him as well, as we were on the way back and knowing Giro, after controlling himself for a long time he was on the edge), before I knew it Kiara had flown down to the dog at top Whippet speed. What I thought would happen did happen: The GSD gave her a nasty reception, fortunately without any consequences, as Kiara had the mind to freeze just at the right time. I'm sure the owner was upset with me for letting my dog go near it. I called Kiara back and left: not much fun to be had today.
So, unless you're actively out to make your dog aggressive: train a fabulous recall, and get out and enjoy life with your dog at the off leash park. But until then, please spare your dog and others the stress of mixed off leash/on leash encounters, just like at the on-leash park.
I started my dog journey with Jessie, a small white fluffball bichon-schnauzer cross. She was trained in the traditional way by choke collar and praise via voice. After she died, Giro, my smooth collie, taught me how wrong this approach was. Kiara, my whippet, reaped all the benefits, and can't wait for her training every day, all day!