So it is impossible to allow JS only on certain websites?
No, if you disable it globally.
Consider it as a car engine. You can't switch on the Air-Conditioner (AC) system without start the engine first.
But, you can do the opposite. You can start the engine, but you can switch off the AC.
Anyway, as Yogi told, read that thread, or better, this post, where I explain how it works