A restore means that iTunes replaces/reinstalls the software and firmware on the device. In order to do that, it needs the image from Apple. In the past, the only way to update your iOS device was through iTunes, so iTunes always had the latest image you had updated to, and didn't have to download it when you wanted to perform a restore. That all changed with iOS 5, which introduced over-the-air updates directly on the device, allowing you to bypass iTunes when updating. If you've been keeping your iPad up-to-date using over-the-air updates (rather than tethered updates through iTunes) then iTunes does not have the latest images on hand, and it must download them before it can perform the restore.
If this is bothersome to you, perhaps you should try "Restore from Backup" instead of "Restore". You can access this option by right-clicking on the iPad in the iTunes left-hand sidebar. This just restores your personal backed-up data to the device and does not do a full reinstallation of the software & firmware.
Alternately, you could try "Erase All Content & Settings" from the device's Settings > General menu, then do a "Restore from Backup" as described above. Make sure you actually have a backup in iTunes or iCloud before you attempt this.
Neither of these two options is equivalent to a full restore, but perhaps you don't need a full restore.
