What if someone makes a typo entering their SSN in the form that leads to this page, are they also a hacker? By your definition, they would be.
If your data is on the internet and not secured, it is being scraped by robots constantly for a start, some of which might iterate counters, so some responsibility lies with those who maintain the website. There isn't a clear line like the threshold of a dwelling we can point to, because it's not always clear which urls are authorised for a user and which are not. Ultimately you're not going to stop the curious, and bots, from scraping the web, so if there are no access controls on your payroll you can expect data to leak, even if you come down hard on every single person you find accessing it without authorisation.
I think the emphasis here should be on intent, as shown by the data taken, and what was done with it, not on trying public urls. If someone shows intent to steal information by changing urls, then downloads the info, then uses it for identity theft or sells it on, that's clearly a crime, and unless they have mitigating circumstances, perhaps it deserves a fine or a very short jail sentence for serious cases. I do think the sentences today are excessive for this sort of activity.
If they simply access a URL as you propose above, I don't think you can show intent. Even if they access several urls, was their intent to explore, or to steal information, or did they just follow a bad set of links or make a mistake with their web crawler?