Re: US company, US law.
You are mostly right, and then again altogether wrong.
The idea is that international companies adhere to the laws of where they operate. Thus if a company wants to be allowed to operate in say Australia, there services would have to stand up to Australian law. Now, things does of course get a bit complicated when things move online. However, the rule does still apply. Most countries have the means to block out an online service if they want to. Thus if Twitter wants to continue to offer their service in Norway, they might have to change their ToS - at least for their Norwegian users. Now, Norway is pretty small country, but it is still always a problem for an international company to be deemed "non grata" in any country.
Figure the annoyance among politicians if they had to attend a summit in Norway and couldn't tweet instead of paying attention to the items on the agenda! - Obviously the same goes for regular users.
I do however not think that it will come as far as this - although I still hope Twitter et. al. will make their ToS more consumer friendly.