I physically removed the SD card and rebooted and yes, it still asked to Check/Postpone (I guess this must be for internal storage). The notification wording could be a bit confusing as it says "New storage mounted. Do you want to check for new/changed media?" - it might confuse some users to refer to internal memory as "new storage"?
My own preference would be to have scanning postponed by default for all media. Or at least make it an option in settings. Maybe I don't really understand the process but I only ever add music to my device via MM sync so I'm not sure I would ever need to have scanning enabled. Is it because some users add music to their device from sources other than MM?