As far as I understand it, the data was publicly accessible and merely visiting the URL output the data he found. I don't view that as hacking.
If you use the word "hack" in front of normal people, they would probably view it as some neckbeard breaking passwords and bypassing electronic safeguards in a dangerous fashion. They would view it as equivalent to that scene from Mission Impossible where Tom Cruise is dangling from wires to break into the CIA or that scene in Terminator 2 where young John Connor gets easy money from the ATM. I use those popular references because that's what most people think is involved when you use the word "hack" when that's not the case here.
This case, if I understand it correctly, literally involved somebody visiting a URL. Whether he did so by typing it into his browser or using an automated program or not doesn't change the nature of the action to me.
However, breaking into someone's car in order to roll up their windows and leave them a note telling them not to do it is perfectly good in my book. So I think it's better when people focus more on the severity of the actual crime committed, rather than on the ease of committing it.
So maybe the analogy works better like this: You're blind and walking around. Suddenly you get to some random car. Being a curious person you touch it to feel what kind of car it is and, in doing so, discover that the windows are rolled down. Reaching inside you also feel that the key is in the ignition. Then the question is, should you go to jail for attempting to steal the car (imagine that blind people are excellent drivers). You certainly did the exact same motions that a car thief would. Which are the exact same motions that a car enthusiast and curious person would do. The analogy breaks down about here, since the guy in the story copied the car by touching it, which we can't do yet.
1. walking away, maybe telling someone
2. sucking up as much private data as you can
Obviously this "hack" was simpler, and AT&T's security was lousy. But the simplicity of an attack is not an excuse for mounting it.
I've written a lot of loops, and incremented a lot of numbers, and none of them have ever become properly escaped XSS exploits, or malformed YAML, etc. If your bar for "hack" is so low that we're training pre-schoolers to accomplish it, it may lack all descriptive power.
URLs are by design human readable and human editable. Simple discovered usage of these resources is not hacking, it is intended.
Imagine you're reading a story at example.com/stories/bears-part_1_of_3.html and just as it gets good, it's done. Without so much as a "The End".
Do you sit dumbfounded, or do you hack the bejeezus out of the website?
See how silly that sounds. So lets just say you "went to the next page".
But it's not the "hack" that you're worried about, it's the stealing. And stealing is separately illegal. Not all stealing is hacking just because a computer is involved, and something that isn't otherwise hacking shouldn't become hacking just because it involves theft or some other malicious activity that would itself be against the law.