Usually the justification for emulation is "the company doesn't sell this product anymore, it's abandonware"
Can’t distribute games or other copyrighted software with them, though. Of course that’s still highly likely to get you in trouble. And with the same caveat as anything legal in the US: it’s as legal as your ability to weather a series of failing lawsuits against you.
Seems like emulation is only fine if they make money out of it.