If you have your dog on the lead in a park it is likely to be due to one of the following:
-Nervous dog that doesn't like other dogs
-Agressive dog that doesn't like other dogs
-Dog covered by the DDA that has to be on a lead
-Dog that will run away or is difficult to recall
-Dog recovering from and injury/illness
-Bitch in heat
In each case, the owner is acting responsibly and the dog has the right not to be bothered by other dogs that are off-lead, even if they're just playful/exhuberant. In the first two cases it can turn nasty for all concerned and, in the third case, would likely lead to the dog being put down. The dog on the lead, especially if a bull breed, will cop the blame even if initially minding its own business until the off-lead dog came into its space. If you're lead-walking a dog as part of its rehabilitation from cruciate ligament surgery the last thing you want is a playful dog getting in its face when you're trying to keep it calm to allow the leg to recover.
I'm all for dogs playing off-lead, but always want to see how they interact first. Reggie is very dog-friendly, but incredibly boisterous, strong and physical and not all dogs like that. There are lots of dogs I will let him play with but I don't let him off the lead in the park until I can see which ones are there, it's for the other dogs' sake as much as for him.
Good recall is crucial and, without it, an unfortunate incident will eventually happen. A few weeks ago, a woman came into the park with two pit-crosses, one off the lead with no collar. It came over to Reggie and tried to dominate him by standing over him, and he was starting to react. She couldn't get it to come back to her and I had no way of getting hold of it. Fortunately a neighbour came over before it got out of hand and took Reggie while I took my belt off to use as a temporary slip lead and took the other dog back to her owner. It was a near-miss...