Making money over someone else's copyrighted contents?
I mean, the Pokémon concept and stuff related are kind of obviously Nintendo's properties, and Nintendo is known to easily sue people for using what's theirs
To show the problem on another perspective, it would be like taking a book, putting your name on it and selling it to people, that's not really nice for the original author who did all the work
edit: PWO already received a letter from Nintendo lawyers about closing the game years ago