There's also the fact that it rings true. Not absolutely true, but in my lifetime I have heard of far more little dog bites than big dog bites happening to real people (as opposed to being in the press). Although the two dogs that have connected with me were larger, I've been SNAPPED at by more small dogs. This doesn't mean small dogs are bad, or vicious. It DOES mean that the perception that they are "harmless" is false.
I would never recommend that someone get rid of their Chi or their terrier when they had a baby, unless the circumstances were unusual somehow. I would not discourage someone from getting one of those breeds (or any other) if it was "their" breed. But when people at the office go . . . we're thinking about getting a dog and we think Jack Russels are cute (to pick at random) what do you think, my response, coming from experience, is "I don't think that's the best breed for a family with small children, in fact I'd recommend something larger." And I grew up with terriers of various breeds, which never bit me. But there is little doubt in my mind (BEFORE reading the survey) that there are better breeds for a household with small children, and those breeders are generally larger, not terriers, and known for their patence and reluctance to resort to their teeth. Which doesn't mean they'll never bite. All dogs can and will bite under the right circumstances, and the vast majority of dogs of any breed will never bite, despite being perfectly able to.
As for whether BSL is additive or subtractive has a lot to do with the motives behind it. If its being pushed by PETA, its additive. If its being pushed by panicked soccer moms it can go either way. It is actually in retreat in some areas as it becomes obvious that it is useless.