As a dog owner I am responsible for the welfare and safety of my dog, and also do my best to ensure that the actions and behaviours of my dog do not threaten the safety or welfare of others. We walk in open countryside. Every day. If I see an approaching dog who is on lead, I put Bingley onto his lead. As others have explained, being on lead autotmatically puts that dog at a disadvantage in terms of displaying warning 'go away, i need space' signals. I do not want my dog to make any other dog feel threatened, as the dog feeling threatened could (justifiably) tell him where to go. I don't want to put my dog in a situation where he could have a bad experience, especially as a consequence of his own 'friendly' behaviour, because his 'friendly' behaviour may not be welcomed by everyone, and I have to respect that. I don't think having your dog off lead gives you free reign to let your dog run up to every dog you encounter, even if you are in wide, open spaces.
We see fairweather walkers, plenty of them, sometimes always keeping their dogs on lead, but frankly it's none of my business why they choose to walk where they do, or why they keep their dog on the lead. On public land, people can walk however they bloomin' well like. Just because I walk my dog there every day doesn't mean I have the right for my dog to have free reign in that area, if fairweather walkers are about. If you don't want to have the hassle of recalling or putting your dog on lead, then don't - but please bear in mind that in doing so you are running the risk of creating hostile, potential damaging, encounters. And to be quite honest, if something serious happens (god forbid) with your dog off lead, and another dog on lead, the responsibilty (and 'blame') lies with you. I don't want to be dramatic or offend, but sorry, that's just how I feel.