Read the DRM section here, specifically:
DRM copy-protection
All cards incorporate DRM copy-protection. Roughly 10% of the storage capacity of an SD card is a "Protected Area" not available to the user, but is used by the on-card processor to verify the identity of an application program that it then allows to read protected content. The card prohibits other accesses, such as users trying to make copies of protected files.
Basically, they're using the "secure" part of the Secure Digital format as it was intended so that only the authorized application (your specific NAV unit) can unlock the partitions of the card and read them. That's why you have to send them the Unit Code and Serial Number of your GPS to order a new card.