Nintendo has confirmed that the time-based age restriction on the Wii U eShop - which prevents 18+ titles being purchased outside of a four-hour window at night - is down to German law.
Speaking to Eurogamer, a Nintendo Europe spokesperson had this to say:
At Nintendo we always aim to provide a safe gaming experience for fans of all ages and ensure that we comply with applicable legal age restriction requirements across Europe. Legal age restriction requirements vary across a number of European countries. Since Nintendo of Europe is based in Germany, Nintendo eShop is complying with German youth protection regulation which therefore applies to all our European markets. Under German law, content rated 18+ must be made available only at night. Therefore the accessibility of 18+ content in Nintendo eShop is limited to [USK: 22:00 UTC until 4:00 UTC] [PEGI: 23:00 UTC until 3:00 UTC].
In case you were wondering, USK is the abbreviation of Unterhaltungssoftware Selbstkontrolle, the Entertainment Software Self-Regulation Body responsible for rating video games in Germany. The UK is controlled by PEGI.