Good dogs can be off-leash. Hyperactive or aggressive dogs should be on a leash. If seeing a dog walk calmly with its owner sends you into fits of terror, your problem isn't the dog.
The problem is everyone thinks their dog is a good dog. Some people think letting their hyperactive dogs run up to and jump on random people is a good trait to encourage, because they consider it "friendly" behavior. People don't know you or your dog. Your dog may very well be good and well-trained, but it's just safer all around to keep a dog on a leash to quickly get it away from other aggressive dogs or other situations (in general or cases of emergency).