Not seeing why people should be that pissed.
It's DLC content, it requires a payment, big deal. If you don't like DLC additions, that's ok. But this is clearly optional content added after the fact, so it's not like they are just throwing in an arbitrary pay wall for Wii U owners.
(That said, it would be nice if the content that didn't actually require saving data to Amiibo would be available for purchase digitally, on all consoles, at some point in the future.)