Because some of the things little dogs do don't have the same result as those same things done by a big dog. If you were to come in my house and sit on my couch, you'd have Chulita (not Jose`) leaping into your lap. She'd come quietly. You wouldn't know she was coming and it might startle you. LOL. She loves people and loves it when someone visits. She would sooner sit on your lap than mine because you're someone new. She wouldn't ask for much...just that she can sit on your lap. Oh...she might give you some licks on your hand too. Jose` would probably go find a toy to bring to you.
Now a big, heavy dog jumping into your lap when you're not expecting it could be problematic. It's a little embarrassing when out of nowhere, a Chihuahua springs up into a visitor's lap. But I don't see it as a terrible behavior. I would never allow it from a bigger dog...even a somewhat bigger dog. But she only weighs 5 Lbs. And neither one of them is hyper or excitable with the exception of when you first come through the door because they think it's just the coolest thing that someone is coming to visit. Then they settle right down.
If you stopped to chat when on a walk and bent down, she'd probably put her feet up on your knees and ask for a pat. Otherwise, just standing there, she wouldn't be apt to...doesn't jump up when people are standing. With my bigger dogs, I wouldn't like them to put their feet up on you. They are too heavy and their nails might dig into you. The Chi's feet are weightless. My niece calls Chulita's feet "little Q-tip feet." LOL.
When people come over, they bark their little heads off. But after a moment, they stop. They're never aggressive, always friendly to everyone. If they're asking for too much attention, I tell them, "away" and they go off and lie down somewhere.
So, imo...the same rules don't all necessarily apply to small dogs as they might to big dogs. There are reasons for that. No, they shouldn't be bothering people or being really obnoxious, but some of those things have a different result than those same things committed by bigger dogs.
Jose` was a bit reactive on a leash toward other dogs...barking, straining at the leash. We use to live where he didn't see that many dogs. But I've been working on it and he is 100% better. He turns his focus to me and hardly barks at all anymore at other dogs. Other dogs are the cue to look at me. I keep him near me when we walk...it's a loose leash walk on a 6 foot leash and if he wants to investigate something, I go with him. He stays within about 3 or 4 ft. of me. I don't go for those flexis where the dogs are a huge distance away from their owner IF other dogs or people are near by. It's fine if you're by yourself somewhere. I see so many dogs on long leashes on the sidewalk where cars go by. How dangerous is that?!?! They could step right out in front of a car!
I see people letting their dogs go into peoples' yards when they walk down the sidewalk. The dogs are on long leashes and they allow them to walk into yards a little ways. I can't believe my eyes sometimes...the nerve! I'm glad I have a fence out front now. But this can be small or large dogs. Someone with a Doberman mix lets that dog walk way up into neighbors' lawns...way up. Incredible!
So, there are dog owners who are clueless, inconsiderate, non-thinking about dangers or about bothering people. They own both large and small dogs. But I think some, not all behaviors that small dogs do don't have the same effect as those behaviors done by bigger dogs.