It's because the actual terms of the exchange are so infuriatingly unclear. Also, because efforts to fix this are met with an endless stream of deflections and evasions, all of which signal an intrinsically untrustworthy character in the organizations making them.
To understand this a bit better, imagine going to the grocery story, buying milk, eggs, etc. swiping your card, and not getting a total.
"Don't worry about that" says the store "as long as we (and our unnamed affiliates) have access to your bank account, everything will be fine."
Going home to look at your statement, you see a bunch of debits, most of which seem reasonable enough on their own, but none of which have a clear relation to specific purchases. All you come away with is a general sense that "I should save more" or "I can spend more".
So here's the question: if people started pointing that this arrangement was highly damaging to people's economic autonomy, and wide open to abuse, what would you think of someone who says "yeah, well, stuff isn't free"?
Would you think that this was an honest, intelligent reply? Or would you note that the person making it has just evaded the original question, shifting the topic away from concern about the deliberately unmanageable terms of the exchange and onto the (uncontested) subject of underlying economics? Also, would you notice that the jerk responding in this fashion added an extra layer of insult by suggesting that the person who asked the question must be a bit of an idiot because they don't understand basic economics?
Toxic politicians do this all the time. They 'reframe' questions before answering, allowing them to 'respond' by answering questions that nobody asked, while dodging the ones they did. If they're especially nasty, the land a rhetorical punch in the process, providing a disincentive to any further questioning.
It's bullshit. And people know it's bullshit. Ergo, the growing outrage.