It depends if you can get access to their account and card details. If i had bought a few games on a Wii U and then i sold it, i wouldn't be upset that other people could download my old games for free. But if they could access my details i wouldn't be happy.
It reminds me of the PS3, if you signed into another persons account, you could view their downloads and re-download anything you wanted on your PS3, which was good to share with your friends.
But i hope the do make the security really tight on it.