When zoomed out, so that the whole image fits on the screen, the pan tool does not work animore. Besides that there seems no logical motivation for this kind of limitation, it actually would be very handy to be able to pan the image regardles of the actual zoom level. Especially when doing transformations, i.e. operating with the transform handles (e.g. perspective distortions) the handles will eventually go way off into the canvas. Now, you always have to zoom out first before you can adjust them further. Generally, when working on edges of pictures it would be great to be able to pan the image so that the area of interaction is centered on the monitor - without the need to zoom in until the pan tool kicks in. When working on large monitors this becomes even more desirable.