*Most commonly the dogs that approach dog aggressive dogs have terrible social skills from becoming magnetised to other dogs by being allowed to play with every dog they meet.
Dogs that are aggressive to other dogs are usually pretty good at communicating that they don’t want to interact, so that’s all being completely ignored if a dog is still approaching.
So yes, dogs with poor social skills and poor recall should be on a lead, they shouldn’t just be part of the environment tbh.*
First of all, dogs learn about communication with other dogs precisely by communicating with other dogs, and not by being "allowed" or not "allowed" to do something by their owner. Owners usually understand much less about dog communication than their dogs themselves, not surprisingly, since they are humans.The idea of teaching your dog about how other dogs communicate aggression is a ridiculous one, sorry.
Second, well you are making some unfair assumptions here. Perhaps it is actually the dog on lead who is poor at communicating? Because, being kept on lead, he was never able to learn how to communicate properly? Or do you think that you as a human can tell exactly the intricacies of the communication between two dogs by observing it from the side? I mean it is something that behaviour researchers are still working on 