Because what you're doing is projecting your opinion on to everyone, and comparing Amiibo to the absolute worst case scenarios for other publishers.
I don't want a plastic toy. I don't care how cheap they are or how easy they are to get or how many games they can be used for or that they can be sold or traded, I just don't want them. I'm in this for the games, and the games only. You say they don't lock away a lot of content but it's obvious from this thread and the length and ferocity of its discussion that yes, significant enough content is locked away for many people to be annoyed. And you neglect the fact that these toys can very difficult to come by, especially for people outside of the US, who don't have the vast network of retailers fighting for their business, offering pre-orders and discounts on these toys.
As for other DLC, as I said you're comparing Amiibo to the worst of practices. The vast majority of DLC around (or at least the ones I buy) are affordable, go on sale regularly and are worth the price they're asking for. I'm thinking of Dishonored's DLC expansions, Dark Souls and Bloodborne's substantial expansions, Mario Kart 8's new tracks and characters - most DLC is simply extra content for games at a decent price. Being digital, there's no chance of a shortage, and they're on global platforms that see universal discounts like Steam. Comparing locking hard mode behind Amiibo to most of the DLC around makes Metroid: Samus Returns and its Amiibo situation look like the terrible deal that it is.
And all of this wouldn't be an issue at all if Nintendo offered options. It's true that we don't yet know if Amiibo is the only way of unlocking a hard mode, but their communication so far has offered barely anything. No one would be up in arms if Amiibo unlocked a hard mode and you could also do it in the game. Options are always, always a good thing.
You are entitled to your opinion on Amiibo. I'm glad you're a fan, and I know people personally who are. What rankles is you trying to come across balanced and fair when you are anything but, disingenuously comparing Amiibo to the worst DLC has to offer.