postcard.co.uk

I have a postcard printed by Davidson Bros in the 1900s which carries the wording: "This space may be used for communication to all countries except United States, Spain and Japan". Can anyone explain the significance of this ie why were these countries excluded?

Divided back postcards were not allowed in the US until March 1, 1907. Prior to that only the address was written on the back and any message had to be on the front.