Memes

Circumventing DRM is illegal under the DMCA, but the DMCA has an exception saying you're allowed to ignore parts of the DMCA if it's for purposes of interoperability between different computer systems. It's that exception that makes emulators legal in the first place. However, there's no case law setting a precedent as to whether the DRM circumvention prohibition or interoperability exception wins when both apply.

That means that the decryption is in a grey area if it's part of an emulator, but definitely illegal if it isn't.

We also don't know if this is an argument Nintendo relied on to stop Yuzu. Their initial court documents claimed things like emulators being totally illegal and only invented for piracy, which weren't true, and they settled out of court, so the public can't see what the final nail in the coffin was. It could simply be that they'd make Yuzu's position expensive to defend with spurious delays until they were bankrupt or shut down and gave them all their money, which doesn't require Nintendo to be legally in the right.

Not long before this, Dolphin's Steam release was cancelled because Nintendo asked Valve to block it, so the Dolphin team double checked they were entirely above board with their lawyers. Despite Dolphin containing the decryption keys from a real Wii, and using them to decrypt Wii games, they were confident it wasn't at risk. The keys are an example of a so-called illegal number, but they're generally believed to not actually be illegal (hence the Wikipedia article about them featuring several examples). The decryption should be safe as the lawyers thought that if push came to shove, the interoperability exception would beat the DRM circumvention prohibition.