Not everybody wants the figures, or thinks they're awesome. Amiibo being physical lowers their value to a lot of people.
While it's true that for some people the cost of Amiibo are spread across multiple games and a figure they like it's not a universal truth. The people who just want these functions without having to pay $15 for a piece of plastic they don't want have every right to be mad because Nintendo *are* asking them to pay $15 and allocate storage to functionality that was previously both free and intangible.
Said this in 2014:
Allow for people to buy the extras that the Amiibos unlock as pay DLC. That way the complains will be about having to pay extra for content that's already in the game like usual. And not make some people "hate" the figures.
However, fault lies in the players for supporting frivolous DLC from the get go and supporting the pay to play model introduced to console by MS. No need to get mad just take your money elsewhere.
Another thing, Amiibo functionality is in a different realm than paying to unlock costumes in other games. The figure is a physical object that holds value in itself, can be lend or sold and has a collecting meaning.
Amiibos so far work a cross many games, systems and now even console generations. There's people that are into collecting independently of the game functionality the figure might have, to them the virtual items that unlock are a bonus. It's Nintendo rewarding their more dedicated fans.
In the end, the majority of cases of Amiibo use are mainly cosmetic stuff. So far is the exceptiuon not the rule, yet people tend to latch on to the negative.