These are iPad apps that I can currently only find in the US iPad App Store.
In some cases, like streaming TV apps, it no doubt has to do with licensing rights. In other cases, like utility apps, it means the developer just hasn't seen fit to make them international yet (glaring at you, Apple!)
Why is iWork US only right now? Can you not buy iWork in Canada? And why are there US-only apps to begin with? Are there US-only computers? I have never understood this nonsense. There aren't US-only cars are there?
My question is, since you can change what store you are in by clicking on the flag at the bottom of the app store, why can't you buy from another country or do they actually check your country of origin?