Pitbulls, despite their reputations, are not more prone to attacking humans than any number of other dog breeds. They do, as a species, tend to be dog aggressive. It should be understandable to pretty much any idiot that a dog would snarl or growl under these circumstances!
I hope that the dog doesn't end up being returned to her prior owner. If she's running around loose, without a collar, I have no high opinion of him/her.