Dog experts attest that dogs that are inherently protective of their "own" property, such as their own house or yard, usually don't adopt a protective attitude when in "neutral" territory. Parks seem to qualify as neutral territory.

Also, dogs with a history of aggression are worse when leashed than when off-leash. It seems that leashes increase the potential for fear and protectiveness, possibly because dogs are more nervous when they are restricted.

This is why owners of difficult dogs often report that their dogs play well with other dogs in parks, but not at home. If and when we have dog parks, we want regulations that prohibit aggressive dogs with documented histories of violence from entering. No dog owner wants their dog to be attacked.