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In an interesting discovery, it seems that games downloaded to a Wii U are connected to that specific console, rather than just your user account. This means gamers who are picking up second-hand consoles are able to access entire libraries of games with just a few keystrokes - even when the console has been formatted and a new account installed.
The "problem" here is reportedly caused by the Nintendo Network ID - that is, a console ID, which is used with Nintendo's online functionalities. According to users on NeoGAF, games - and other content - are linked to the Network ID rather than the User Account, highlighting a bit of an issue within Nintendo's system.
Gamers are also reporting similar things happening on the original Wii and the DSi handheld.
At this stage, Network IDs are non-transferrable between consoles without intervention from Nintendo proper. The company has previously announced plans to update the Wii U firmware to give this functionality to end-users, but no timeframe has been provided.
For now, if you buy a second-hand Wii U, check out the Store and keep an eye out for the "Redownload" option. Just sayin'.