Because the 3D view button obviously shouldn't be the button for moving the 3D view! Oh heavens no, it is so simple and unintuitive!
It may seem obvious once *you know about it*, but with no instructions anywhere saying that's what to do, it is something less than obvious to a normal person. Face it, their control scheme svcks, and the controls documentation svcks even worse. Damn console-happy crap. And you can't even remap key things like the weapons keys. From a controls intelligence and ease-of-use scale, it's one of the worst games I've ever played. Luckily, the game itself makes up for the cruddy controls.