If the DLC is free and available from day 1, why make it DLC to begin with? Why not include it on the game card?
To force more people to buy the game right away at launch instead of waiting a few months to buy it cheaper or used. Unlike in North America were Nintendo can keep the price high for years, in Japan stores will lower the prices for Nintendo sometimes after just a few weeks if the sales aren't to hot, plus the used game industry is pretty huge in Japan as well.
Basically this will be Nintendo's way to try and maximize profits in Japan by making people either buy the games at full price for free DLC or buy it cheaper or used later on but will have to pay Nintendo for the DLC so Nintendo will still end up making around the same amount of money with each sale.