I sort of get both sides here. I understand why people don't want to be approached by loose dogs that they cannot predict the behaviour of, but also think that dogs shouldn't be kept on the lead all the time or repeatedly put back on the lead every time they see another dog as this in itself can lead to all sorts of problems
All three of my dogs have been trained to respond to the 'leave' command and that means leave be it a toy, person, food or another dog, so even if they are off lead, they don't approach other dogs or people unless I've given them the ok first.
I think someone else mentioned it being an urban problem. Dogs can only learn to communicate with each other if they are allowed to interact. Unfortunately not enough dog owners start socialising their pups from a young age and the result is lots and lots of dogs being walked in public places that aren't able to read each others body language. Some of these dogs will react in an aggressive manner either through simple misunderstanding/misreading of the other dog's body language or through fear/anxiety or dominance. A sad sorry state of affairs when you consider how peacefully wild dogs can live together communicating in the main through this very medium.
These days many dogs are very often kept in isolation from one another and walked once a day around the roads or park whilst attached to a lead. Dogs are intelligent animals that need social interaction with others of their own kind, mental stimulation and exercise. Dragging them on their leads on the same route around the park or roads day after day, simply does not meet their needs and we are often left with socially inept animals who are also either pent up through lack of exercise or depressed through lack of mental stimulus.
Often a dog that would not be aggressive in any other situation, will react aggressively on the lead as they cannot escape and with the 'flight' option removed they have only the 'fight' option left to apply. This can be made worse by the fact that every time an owner calls his/her dog away from another dog and puts him back on the lead, he receives reinforcement of the message that the other dog is in fact something to worry about thus heightening his anxiety and increasing the changes of him reacting in a negative manner.
I am not saying that people don't love or care for their dogs, because in the vast majority of cases they do. Unfortunately in a lot of cases there is just a lack of understanding of what exactly a dog needs to keep it physically, mentally and emotionally healthy.
Ahem. Anyway - I shall get orf me soapboax now and try to answer the OP. YABU yet YANBUish. In an ideal world, dogs would be able to run and play together off lead with very few problems, unfortunately in the current climate of undersocialised, understimulated, underexercised dogs, it is sometimes advisable to exercise caution when approaching other dogs. Imho there is no 'unwritten law' about putting dogs on lead when meeting other dogs on a walk, it is really down to individual assessment of the situation, the other dog and the owner. I understand why you might feel the need to put your dog on the lead to keep him with you while you are running and as long as you let him socialise with other dogs on other occasions that is absolutely fine. However, it would be much easier for you if you trained him to come and leave on command, then you could just carry on running, give your dog the command and there would be no need for you to stop and put his lead on.