Gotta read between the lines on this; look at the fact that it doesn't work in both directions in the European release, and the fact that they call this "conditional purchase" instead of something more similar to cross-buy.
Assuredly what's happening here is that Nintendo doesn't actually have a single store system with a concept of products and entitlements; they have two separate systems for 3DS and Wii U, which are only linked to a shared identity provider (the NNID).
The way this is probably implemented, the 3DS system sends a message to the Wii U system (that they probably had to set up manually) after a purchase and tells it to add an item to the user's download history.
It probably doesn't work the other way (Wii U -> 3DS) because the 3DS system is older and isn't set up to capture triggers the same way. It probably doesn't work in the US because they're running on a completely different version of the store infrastructure and it hasn't been updated with the conditional