Probably on the part of the map card makers. They do not want multiple units able to use one card. (shared)
I think you can still load the card individually but seperately over multiple units..Just not at the same time..
Just as we cant copy the card to another sd..
Lesson #12 in economics..
I would think the code to share a map card between units would be very line intensive for the unit and
may have to do with ownership of the card manufacturer.
Now having said all that. It would be nice to have one card shared between units..

Chuck