The only reason a screenshot (or a default image) is used is because of the way applications are launched. They expand from the center of the screen, and the image expanded cannot be a live view of the application. This is opposed to Android, which uses a fade animation to launch applications (at least on ICS).

It is not deceptive, it is smart. Android applications appear to have a delay in launching because there is a moment of inaction after tapping an icon. Apple made it so that immediate action is taken after an icon is tapped, and that does make it more responsive.