The policy came to light after NeoGAFer Panajef2001a tried to access the Italian store during the day, only to get an error message. When he wrote in to ask, he got the following message back from Nintendo:
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
We at Kotaku are all either stateside or across the Pacific, so we can't try this out for ourselves, but Eurogamer gave it a whirl and got a similar error message: "You cannot view this content. The times during which this content can be viewed have been restricted."
So, if you're in Europe, you can't buy mature-rated games (like the outstanding ZombiU) during the day. Even if you don't have kids and are a grown adult capable of making your own purchasing decisions. Very strange.
I've sent off a note to Nintendo asking for clarification on the policy, and will update if I hear back.