It's not illegal to play games that you already purchased and own. Piracy is just a convenient scapegoat to hobble emulation.
Zelda BoTW being released 7 days BEFORE being on Switch, and 60FPS, and better gameplay on a real computer is what caught Nintendo's attention.
And with Yuzu and joycons, you can have EVERY game, including their online only stuff. And if you pay for those online only, piracy is the only way to guarantee that you'll keep them.
They've destroyed the: NDS, Gamecube, Wii, WiiU stores/online content. They never needed to. They *chose* to.
Thinking further on it I wouldn't write "Nintendo" either: "yuzu is a Switch emulator that runs your games on PC/Mac/Android - often with better resolution, framerates, ..." is fine - people already know about the games they want to play.
It's illegal to pirate games. Being able to emulate things you purchased is a convenient scapegoat for people to pirate games.
Why should I not be able to emulate them?
Well, there's a bunch of places where that's not true. Then it's a question of morality, although that usually gets unfairly asked of the individual but never asked of giant mega corporations.
/s