Incorrect.
The emulators themselves are not illegal, since they violate NO copyright. Think about it-- you can make a "knock-off" product, we see them every day from the Chinese, and not violate copyright laws. The code of the emulators is copyright the authors, NOT Nintendo.
What is probably illegal, though, is owning ROMs without having the source cartridge. Therefore, YOU do not own the program code on the ROM cart/file. Nintendo does.