Game system emulators aren’t illegal, as has been proven in court multiple times. Using copyrighted code (be it bios, firmware, or ROMs), on the other hand, is.

For the last time, there’s been multiple US court cases, and while companies would absolutely LOVE for emulation to be illegal so they could talk like you, it isn’t. Emulators are totally legal. Pirating ROMs isn't and that's definitely spurred on by emulation, but emulators are totally legal.