I wouldn't be surprised if they cut a deal with Rare to get King K. Rool, but maybe they actually own the rights? And I don't actually mind all the DLC for Mii Fighters, 'cuz you can just not buy it if you don't want it, and I don't think it's terribly expensive anyhow.
I was kinda sad that they added Splatoon Mii Fighters because it means they won't add Squid Kids as a full character :P they'd be so cute, too.