Sweeping statement to start there as its certainly not all people who let their dogs off the lead.
I let my dog off the lead at every opportunity. He loves to play, and has never been vicious or aggressive to another dog or person (only me and my partner when he is testing as he is still a pup). But, if a dog is coming towards us on a lead, he is immediately put back on his lead despite the fact he is desperate to say hello and play. He has had off lead dogs go at him and try to bite him, and he still doesn't react, he just tries to get back to me.
My issue then is, there are plenty of places to walk, if you cant/won't let your dog off the lead, why take it to areas where its common for dogs to be off lead?