SuperNova on 17 May 2017
With Splatoon it was something born out of neccessity, they pushed that game out unfinished and had to update it for free to make the purchase worth it. It happened to work as a hype and marketing machine for Splatoon, keeping the talk about the game constantly around. It seems like Ninteno reconized that they stumbled onto a marketing goldmine.
ARMS honestly seems pretty complete at launch, so it's extra nice to see them include free updates.
