"A publisher has to protect its property, which is why Nintendo sues/litigates fan game makers." It's a phrase you see often, especially in defense of Nintendo, but it's not quite true in all cases, is it?
Why didn't Capcom lose the rights to the Mega ManIP when Street Fighter x Mega Man was created, among a litany of other fan creations? I must have also missed the news about Sega losing the Sonic IP to fans after the 1000+ Sonic fan games created every year tore it asunder from Sega's cold dead hands.
While copyright/trademark law (difference) is a mess, many publishers have found a way to transmute a curse into a blessing. Nintendo, just like they were with the now-defunct "YouTube Creator's Program," are behind the times with fan games.