It's both the hospital and insurance provider's fault. In my opinion it's absurd that non-emergency care costs can be adjusted after the fact.

Imagine if a car repair shop could quote you $500 for a fix, then after the repair is done, you've paid, and you get your car back, they say "actually it's $5000, and we'll send you to collections if you don't pay." Non-emergency medical services should be required to follow a similar model, and none of the vague "you agree to pay far, far extra if we don't think to tell you about it ahead of time" bullshit that pretty much everyone has had to deal with.