Emulators are perfectly legal in the US. What is illegal is the games themselves (ROMs and the like) + any type of copyrighten bios (like the ps1/ps2 etc) You can legally download a PS1/PS2 emulator and dump YOUR bios from YOUR console to use with the Emu, as well as use the games you own on the emu. If anything, they would go after Apple for selling this Emu, not for allowing it to be a usable app.
EDIT: From Nintendo's side, the only thing they really can do is not allow Apple to sell their games without their permission.









