Quick translation: "Dear customer, we would like to let you know that Nintendo has always aimed to offer gameplay experiences suited to all age groups, observing carefully all the relevant regulations regarding content access that are present in the various European countries.
We have thus decided to restrict the access to content which is unsuitable to minors (PEGI) to the 11 P.M. - 3 A.M. time window [...]"
...by making the experience for anyone not being a child as painful as possible.
TV content rated 18+ is also limited to 11pm to 3am, at least on German free to air stations. The limitation isn't in place because "lol Nintendo", they're simply following laws here. Granted, those laws were made for TV, but since there are no laws governing digital distribution, it's a gray area. Better to be safe than sorry.
But on eShop you are not a passive observer - you don't turn on your WiiU and are bombarded with images of graphic violence. you have to navigate to eShop and then to 18+ game area. This is not the same as turning on the TV.