It's readily evident that people have an account with a default password on the site for some amount of time, and some of them indefinitely. You know what data is in the account (as the person who creates the accounts) and you know the IDs are incremental. You can do the login request and never use the retrieved access/session token (or use a HEAD request to avoid getting body data but still see the 200 OK for the login) if you want to beat the dead horse of "there exist users who don't configure a strong password when not required to". OP evidenced that they went beyond that and saw at least the date of birth of a user on there by saying "I found underage students on your site" in the email to the organization
If laws don't make it illegal to do this kind of thing, how would you differentiate between the white hat and the black hat? The former can choose to do the minimum set of actions necessary to verify and report the weakness, while the latter writes code to dump the whole database. That's a choice
To be fair, not everyone is aware that this line exists. It's common to prove the vulnerability, and this code does that as well. It's also sometimes extra work (set a custom request method, say) to limit what the script retrieves and just not the default kind of code you're used to writing for your study/job. Going too far happens easily in that sense. So the rules are to be taken leniently and the circumstances and subsequent actions of the hacker matter. But I can see why the German the rules are this way, and the Dutch ones are similar for example