Amazon's decision to shut down its grocery stores' flashy Just Walk Out technology delivered a slap in the face to some of the most extreme prognostications about AI.

Why it matters: AI is still not ready to operate on its own in complex physical environments full of people, like grocery stores or roads.

Driving the news: Amazon is phasing out the Just Walk Out system in its full-size Amazon Fresh grocery stores. That's the tech that lets shoppers bypass checkout lines by tracking their purchases with cameras and sensors.

• Instead, shoppers will use a "smart" cart that scans and registers each item as it's added.
• The Information first reported the news. Just Walk Out will continue to operate in Amazon Go convenience stores.

Between the lines: The experiment in cashier-free stores offered convenience, but the data-obsessed tech giant seems to have concluded that it wasn't improving at a fast enough rate to make it cost-effective, experts suggest.

How it worked: Just Walk Out — like many AI systems — relied a lot on old-school human labor.

• Amazon used workers in India to label the data that trained its object-recognition AI. They also served as backup reviewers for problem transactions.

The AI industry relies heavily on cheap labor, often in developing countries, for data labeling tasks.

• AI experts regularly argue that most of today's systems won't work accurately and safely without a human being in the loop.
• Human reviewers rate AI responses, and those ratings are fed back to the system so it can "improve."
• This technique requires a lot of humans at the start. The system is supposed to keep improving until it no longer needs human feedback.

What we're watching: AI may not be ready to make supermarket checkouts obsolete, but there's a silver lining: If it's not capable of tabulating a grocery bill on its own, it's also not able to destroy humanity.