Don't want to give iMovie for iPad the ability to know your location? Well then iMovie for iPad won't let you see your Camera Roll. No, it's not being petulant it's probably just an odd result of the way iOS handles permissions.
Camera Roll may contain geo-location data in the pictures and videos you've taken. If iMovie accesses Camera Roll, it gets access to that location data. My guess is that Apple feels it then needs permission to "see" the location data.