I think it's the level of danger involved. My 100 lb dog could seriously maim or kill someone versus most cat scratches which hurt but rarely require medical attn or do lasting damage. It's the same reason so many little dogs are allowed to be bitey jerks and people think it's funny.
That's pretty much it. A cat bite is something to be a bit concerned about, but a scratch? Unless you're immunocompromised it's not normally a big deal, paper cuts hurt more. Horses and big dogs, that's another story. Ppl are more concerned training big animals, than small. Well, usually.