It was possible to build emulators legally using clean room design but capital found a way to clamp down on it.

The DMCA has a provision U.S.C. 1201 that bans any breaking of "anti-circumvention" measures. So it's nominally legal to reverse engineer the code but it's illegal to first decrypt that code in order to reverse engineer it effectively making reverse engineering illegal.

Pluralistic is a good blog on the subject and I wouldn't be surprised to see an article about ryujinx show up soon.