Many dogs are being removed from their homes just for the way that they look, responsible dog owners take measures to ensure their dog is well cared for and trained properly. Responsible dog owners already have their pets microchipped and insured, making this a blanket requirement is unnecessary.
Some people may provoke a dog into a defense attack, either for fun or, particularly in dog fighting circles, as they are rivals with the owner. Dogs should be allowed to defend their homes as it is a natural instinct. When frightened or cornered some dogs will naturally try to defend themselves, especially if they have been mishandled, this may also be the case when a dog has escaped from its home and has been chased and caught by strangers.
Restricting the movement of dogs in open spaces decreases the available socailisation to other dogs and people, therefore making dogs less able to communicate with other dogs and people and therefore more likely to defend themselves if they feel confronted. In areas when local councils have banned dogs from open spaces there is the danger that there could be more dog attacks as the dogs have become unused to other dogs and people.
There is a lot of scientific evidence regarding how dogs are managed and the effects it has on their behaviour, especially the studies at Bristol University.