Nintendo has always been viewed as a family friendly company, which is a primary reason why they've been so late to the game as it pertains to adopting an online infrastructure. So a recent revelation, as it pertains to their European territory, might hardly be a surprise to some.
Nintendo has reportedly begun blocking access to content that is rated 18+ on the Wii U eShop. Games like ZombieU and Assassin's Creed 3 can only be purchased between the hours of 11pm and 3am. Even associated trailers are not viewable outside the four-hour window as well.
For those wondering if this is only applicable to those with parental controls are mistaken. This applies to all accounts, across the board, throughout Europe.
As for the root reason, Destructoid believes it has to do with Germany laws, which are especially restrictive when it comes to all forms of entertainment, not just games, that depict violence.