They're not free because you paid for them when you bought the game. With AC, Nintendo is following a model that's similar to the "games as a service" model that many games are using nowadays, but AC's case is exactly the same as Street Fighter V's. Instead of them giving you the entire game at once (like it normally happens when you buy a game), they first give you the basic framework of the game, and they'll slowly give you the rest. All that content is something you already paid for, but it won't be handed to you at the moment of purchase. The updates aren't free because they're actually part of what you bought. They're not doing any charity here, they're simply giving you what you paid for.