According to Nintendo, quite a lot of things related to even emulators can be illegal
Some direct quotes from Nintendo:
A Nintendo Emulators is a program designed to play games on a platform that it was not created for. A Nintendo emulator allows for Nintendo console based or arcade games to be played on unauthorized hardware. The video games are obtained by downloading illegally copied software, i.e. Nintendo ROMs, from Internet distributors. Nintendo ROMs then work with the Nintendo emulator to enable game play on unauthorized hardware such as a personal computer, a modified console, etc
There is a good deal of misinformation on the Internet regarding the backup/archival copy exception. It is not a "second copy" rule and is often mistakenly cited for the proposition that if you have one lawful copy of a copyrighted work, you are entitled to have a second copy of the copyrighted work even if that second copy is an infringing copy. The backup/archival copy exception is a very narrow limitation relating to a copy being made by the rightful owner of an authentic game to ensure he or she has one in the event of damage or destruction of the authentic. Therefore, whether you have an authentic game or not, or whether you have possession of a Nintendo ROM for a limited amount of time, i.e. 24 hours, it is illegal to download and play a Nintendo ROM from the Internet.
Emulators developed to play illegally copied Nintendo software promote piracy. That's like asking why doesn't Nintendo legitimize piracy. It doesn't make any business sense. It's that simple and not open to debate.
U.S. copyright laws state that copyrights owned by corporations are valid for 75 years from the date of first publication. Because video games have been around for less than three decades, the copyrights of all video games will not expire for many decades to come.
Yes. Personal Websites and/or Internet Content Providers sites That link to Nintendo ROMs, Nintendo emulators and/or illegal copying devices can be held liable for copyright and trademark violations, regardless of whether the illegal software and/or devices are on their site or whether they are linking to the sites where the illegal items are found.
While there maybe a loophole to using hacks with patches such as .IPS and xdelta etc. Getting a ROM for the Hack to be patched in the first place is illegal. We're all doing something illegal here. But, I don't really care whether is legal or not.