I think Nintendo cares a lot. In fact I think Nintendo gives so much fucks about its IP (of any kind, not only games/stories/characters, but also SDKs/APIs/cons

I think Nintendo cares a lot. In fact I think Nintendo gives so much fucks about its IP (of any kind, not only games/stories/characters, but also SDKs/APIs/console design/etc) that Valve fears any kind of association with potentially illegal or at least dubious origin software related to Nintendo. Remember Valve is a company that normally celebrates and promotes the modding community, even when using their IP directly (i.e. Black Mesa). Valve's decision was totally and exclusively because it included BOTH their and Nintendo's IPs in the equation.