I imagine that the database is enormous and constantly changing, so it's probably impractical to have development copies that are sufficient to test your code against realistically. I'll bet Google engineers have access to their full database too for the same reason.

On Reddit, someone mentioned that 'becoming someone', which I presume means making the software think you've logged in as that person, is unusual and must be justified and closely tracked when it happens. So I'm guessing that you can't just pull up records for some person from the database; you need to 'be' that person to get their data. The free access is probably just for stuff that is shared via the privacy controls.