> If someone posts a video that is obscene
I live in the USA. Barring a few very specific categories I'm allowed to post almost anything. At worst I can get sued for refusing to take it down.
> And if someone does this from another country then most countries have extradition treaties in place.
What? I can violate all the New Zealand laws I like. I live in a sovereign nation very far away.
Don't forget that most of these sites have terms of use which explicitly bans the uploading of these types of videos. So even if you live in a country that permits you sharing it, you'll still need to find a site willing to host it.
Similarly if someone murders someone it's a crime when the event happens, even if it is not found out until later.
In all cases ignorance of the law is not a defence.
And if someone does this from another country then most countries have extradition treaties in place.