Seriously? I guess it's common sense among people living in the U.S, but as an international student I honestly didn't know it's legal to emulate a proprietary system.
At the moment, there's a game compilation being sold on the Switch and other platforms called Disney Classic Games Collection. It contains emulators and ROMs stripped of the trademarks of their host systems, with the emulators created without the involvement of them too.
Nintendo can't do anything about third parties emulating their hardware without also threatening the legal status of software released by their partners, which include Disney, but that's just one example. Suffice to say that even if you argue that Yuzu/Ryujinx step over the line, the foundations for all this are fragile enough that no one wants to rock the boat.