The fund does not invest in larger stakes than about 10% per company, I believe, so that would mean pissing off a whole lot of their own citizens too, to obtain amounts adding up to a quite small percentage of their own economy.

There's no reason (once you've decided to nationalize) that you couldn't just nationalize foreign interests (or even interests held by a certain (group of) nations -- such as a nation you're at war with?

There's some safety in being entangled with everyone else. It would be difficult to nationalize just Norway's stake and avoid upsetting other nations.