If someone sends me a document with text in it that they meant to remove but didn't and then I read that text, I haven't hacked anything they're just incompetent.

Hacking is unauthorised use of a system. Reading a document that was not adequately redacted can hardly be considered hacking.

Or in case some folks find the addition of a computer confusing here, if someone sends you a physical letter and they've used correction tape or a black marker to obscure some parts of the letter, and you scratch away the correction tape or hold the letter up to a light source to read what's underneath, have you committed a crime?

I'm not a lawyer, so I don't know what the law has to say about this. But I do have at least a small handful of brain cells to rub together, so I know what the law _should_ say about this.

Hacking is not just authorised use of a system. Hacking and hacking techniques can apply to systems you fully own or systems which you are authorised to hack. Hacking is using something in a way that the designer didn’t anticipate or intend on.