Sally, it's a staffie and although they do get a bad rep. I've met several who are lovely, sweet dogs
and none that are nasty 
I'm Staffordshire born and bred - and when I were a lass, a dog like that was not a Staffie it was a Pit Bull Terrier. (I've got all the old Observer's Book of Dogs from way back so I can prove it too!

)
Now I know nothing personally of any Pit Bull Terriers, and I do know and have known many delightful Staffies, so I wasn't meaning to suggest anything other than that the judge may have thought, "That's a Pit Bull, it's been off on wanders before, it's done a whole lot more than just
chase a cat, I'm going to save us all a whole load of grief here."
On the subject of chasing cats, yes most dogs chase cats. Not many savage them ferociously, though. That's either breeding or training - and in either case, as a cat lover, I have to think the judge maybe has a point - if the owner will not constrain the dog, then sadly it is the dog that will have to pay the ultimate price for its owner's negligence. The alternative is a lot of innocent family pets paying.
Loads of posts while I had this 'in production', so it sounds like it probably is a bad dog with bad owners.
I have greyhounds and whippets. A cat wouldn't stand a...well.. a cat's chance if they saw one whilst off lead (which they never are except on our own land.) Would you class a whippet as a dangerous dog?
Sylvia, that's the point - you know you have killing machines so you take great care about when and where they are loose. This owner clearly doesn't and hasn't.