It is. People like to talk about copyright and sure that is part of it. But the main thing they are worried about is their Trademarks. Like, the name "Kong" used to be protected and owned by a movie company until they were defeated in court by, guess who, Nintento. Thing is: if a name becomes associated with a certain concept, like if any game that is "game where you catch monsters" is, in the public eye, perceived as "Pokemon" it'll open the case of whether or not it should be allowed for anyone who is not Nintendo to create a "game where you catch monsters" and call it "Pokemon" and allowed to make money off of it.
Given how popular the series is you can be sure that many competitors with a very high budget (Microsoft, Sony) are just waiting for the opportunity to make profit off of it. Nintendo may be big, but it's nowhere near as big as those two especially. They also only have video games as their main source of revenue.
