Landrover - you obviously have no idea what it means to rescue a dog, what a rescue centre is or how they work. DBF does, and can post with authority on the subject, you cannot.
You say "The problem with rescued dogs is you have absolutely no idea what has gone on with their upbringing. You are letting people adopt dogs that could do anything". Any dog can do anything, any animal you invite into your home could do anything. My MILS cat was an evil bitch from Hell and we were all far more scared of that animal than any dog I've met!
You also say "so they spend the rest of their life in a kennel?". No, they dont, they could spend the rest of their life at a rescue centre, that doesn't mean kennel. They could be fostered by someone attached to the rescue centre, that doesn't mean a kennel.
Oh and OP, lots of dogs are agressive to other animals, not even necessarily other dogs, but it does not mean that they are aggressive towards humans. My wonderful, stupid, soppy Mutt detests squirrels and would tear one apart in seconds if she caught it. She loves humans (and ferrets, strangely).
(Oh and a Royal Mail postman can refuse to deliver to a house where a dog is barking if he 'fears for his safety' so it certainly does not mean that that dog has bitten a postman)