I mean, if you go to a restaurant, a movie theatre, or a supermarket, do you know how NOT to be an asshole? It's not hard. Most people can manage it on a day-to-day basis.
I don't know where you're veering off to in your last sentence either. Weird.
Of course most people can manage not to be an asshole on a day-to-day basis, but to deal with the small minority of bad apples who could ruin a whole barrel, it helps to spell out specific banned behavior in detail to quickly shut down arguments about whether a specific behavior is being an asshole or not.
The last sentence is pointing out that just because you've only ever seen someone be a nice person, doesn't mean they've never been an asshole when you weren't there.
Also, I didn't write that sentence. Everything after the first sentence is quoted from the link I provided.