Can't turn down yet another opportunity to squeeze another dime out of their loyal fanbase. Times are hard.
You say that resentfully, but you forget Nintendo has been Losing money.
Oh, I know. Times are hard.
But sometimes you gotta give a little to make some headway. Even though this game ultimately means nothing on a grand scale, the gesture of same game on 2 platforms for the price of one might potentially get more people to check it out as they feel they get more for their money. How many people are really going to buy the same game twice?
If they don't want to give it for free, then charge a small premium.
$9.99 for the Wii U ~or~ 3DS version. $12.99 for the WiiU & 3DS version. Like the Blu-ray w/ DVD &/or Digital Copy. The catch more flies w/ honey, by sweetening the pot approach.