I understand the need to walk near other dogs, to socialise and retrain, but I would never in a million years have put Devil Dog through the ordeal of walking across an off lead area while he was leashed during his fear-aggression training.
I was walking with my Dad and his dogs (and mine) earlier and one of his -who is always off in the distance somewhere- ran up to a leashed dog, now I totally agree with everyone who is going to come along and say his dog should be trained better and should not be off leash until he is, this is an argument I have had many times with him. I have suggested training techniques and even offered to buy the tools he'd need to train (e.g whistle, treats, longline etc) but he's not interested.
However surely the couple with the aggressive dog were being unfair to their dog walking him in an area where it is common knowledge that 99.99% of the dogs there are off lead and will play together?
There are multiple on lead parks and country walks and beaches around here. Why pick the off lead one and then complain when dogs are off lead? 
My Dad chooses this area to walk in 1) because it is quiet and he often goes for weeks of walking there nightly without seeing another soul and 2) because it is the accepted off lead part of the beach where most people are happy for their dogs to play. He'd never dream of letting his dogs off lead in area where it is both on and off lead and more control is expected because he doesn't have control over that particular dog.