I really dislike people who can't control their dogs yet let them off the lead anyway.
Back home we have 2 terriers. (Unless there is a biscuit or a warm lap to take advantage of

) soft as sh*t and now spoilt but they are still terriers, 50% instinct 50% thick, for at least part of their lives they were used as ratters. The problem is they work as a pack and together they can be aggressive and territorial towards other dogs. We walk them on long flexi-leads to they can run around but they are under control, (we tried dog training but the conclusion from all was they were too thick)
However when someone is walking their dog in the same field the natural response is for the other dog to come and have a look. Great, if you want a chewed dog, they aren't fussy and will pick a fight with anything, staffies being a favorite, and at least one of them would come off worse.
Some owners are great, shout at their dog, dog goes back to owner, but some...
"Oh he's alright mate he won't hurt you"
"I have what appears to become a pair of slavering wolverines
on leads here, get yours on a lead now!"
They haven't killed any pets (yet) but rats, squirrels and the odd fox have discovered that even stumpy terriers can be quicker than you think...
If you can't control it off the lead, then don't let it off the lead!
/rant