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Some Walmart employees say customers are getting hostile at self-checkout — and they blame anti-theft tech
(www.businessinsider.com)
This is a most excellent place for technology news and articles.
They've just recently replaced all their self checkout stations with new ones that do that, so maybe the ones near you are still the old ones.
They actually just installed a bunch of new stations in the Kroger closest to me, so I'm reasonably certain they aren't old. The ones they installed don't do what you're talking about though.