I'm a bit surprised it costs extra. Didn't Nintendo at some point say they won't be charging for DLC?
No.
"We would like to supply consumers with only add-on or downloadable content which they are happy to pay for as compensation for creative work. For example, some might say that it would be unbelievably profitable to provide paid add-on content for "Animal Crossing: New Leaf," but we were concerned that a game in which you enjoy yourself more by the power of money would not be suitable, and we decided to avoid such a feature after an intensive discussion with the development team. On the other hand, we are selling sets of new courses for "New Super Mario Bros. 2" by download and obtaining a very good response from those who have actually bought and played them. We see vast potential in this sort of add-on content. In short, we would like to find the type of add-on content consumers will find enjoyable in each game. We intend to basically create add-on content with which we will be able to make long-term relationships with our consumers."