"Legal age restriction requirements vary across a number of European countries," a Nintendo rep told the site. "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." The rest of Europe just suffers from collateral youth protection.
Nintendo of America is subject to no such legal concerns, and thus there are no timed content restrictions on the eShop.