Mobile Units limited mobility

Mobile units are less helpful than at first sight. Although the inference is there that you can elect a country they can be lent to, it seems the lending is limited only to a federation member or a member of your own empire. A wider mobility across federation members and their members empire countries would be more realistic as they'd typically be friendly states. Was this an oversight?

Why can't players offer mobiles to any nation they choose? Is there a particular reason?