Realistically, what did anyone expect them to do?

You want them to sue the US government? For a song that, while technically copyrighted, is so prolifically used that they’d be laughed out of court?

a zillion musicians already have.

You act like most lawsuits in the country aren't against the US government. It's in no way unique or unusual thing.

Nintendo had literally nothing to gain from fighting this.

Oh well, give them something to gain for it than. Such as pegging them to a fascist hate-filled regime and harming their family friendly PR.