Certain information becomes part of the Nintendo Network ID after registration. It is especially important to note that saved credit card information becomes saved as part of your Nintendo Network ID.
Saved credit card information within Nintendo eShop
If you register the same Nintendo Network ID on both Wii U and Nintendo 3DS, the saved credit card information from the system on which the Nintendo Network ID was created will be transferred to the second system. In the instance where a Nintendo Network ID was created on Wii U, saved credit card information from Wii U will be transferred across to Nintendo 3DS after registering the same Nintendo Network ID on both systems.
Please note: Although it is possible to save credit card information in Nintendo eShop, a password is still required to purchase content.