Technically it wouldn't need to remove all copyright material (since that would be everything graphical or textual), The Aaron. (Even one's own original drawings with Roll20 drawing tools are automatically copyrighted at the moment of original creation, that's just how copyright works nowadays.) Only needs to remove the copyright material for which you don't have permission to reproduce.
If there was a "Share map" button, and if this button removed any Marketplace assets for which you don't have permission to reproduce, it could be used (well within proper rights) to distribute copyright material for which the copyright holder -- namely the creator / artist -- gives permission.