my wifes boss also bought this for their kids, along with the Wii at xmas (bundle) - it ensures that younger kids also has an interest in the Wii and if a console is gonna be this gens number 1 then it needs diversity.
I get your point about devs looking at this game Man but you also have to consider is how many of those million sold will have been at either a discounted price or bundled - it's not a full price game in my local shops.
I also see it at Tesco's supermarket when I go there and it's very good for Wii brand awareness to a certain demograph of people.