It's just one of those technical legal things. Age/weight/some such difference. We were a Kosher slaughterhouse, so I can't speak to if pork ever got in - I'm guessing not in the Kosher dogs.
I was such an adorable teenager. Occasionally we'd get a call from someone who wanted to know if we could slaughter a pig for them. I'd put them on hold and then page one of the Rabbis. He'd take the call and then call me back: Joanie! You know we can't slaughter pigs here! I'd giggle and he'd sigh. I liked him a lot.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine