I see what you're getting at, but there are many things that the US government would love to prevent you from saying that the Chinese or UK government couldn't care less about.

For instance, if you're a US citizen and a resident of the UK or China, you might want to call the President of the US a big baby on twitter. The UK or Chinese government probably wouldn't have a problem with that, but the first amendment protections you enjoy, even while abroad, is preventing the President from calling up twitter and trying to have you banned.