At a basic level, it's because her mental model of How Things Work broke. She thinks that it's reasonable that using the Amazon Kindle application on any platform she ought to be able to buy books from Amazon as part of that app. After all, she's signed in, Amazon knows who she is, at most they ought to ask for a password confirmation to let her spend her money.

And that doesn't work.

So she was unhappy with Amazon. Reasonable. She was so unhappy, she posted on Facebook about it, where a dozen of her friends pointed out that Apple wanted a 30% cut of content sold on their device.

Their device. Not her device. She paid for it, but now she sees that it isn't hers.

She thought that an iPod was like a computer: you use it, you select software to put on it, you use the software.

Her mental model breaks AGAIN, because she thought she owned the iPod. Now she finds out that it's actually an Apple-owned store that doesn't like competition.

Apple got their benefit when they sold her a device. They got more benefit when they sell her software to go on the device. Why should they get more benefit for interfering in a transaction between her and Amazon?

She knows what rent-seeking means, and she doesn't appreciate it.