> the machines are set to "complain about a fly-landing on it" sensitivity and regularly berate me for not scanning everything even though I do
This is partially because they've implemented that flow in the worst way possible. Do not stop me from scanning every time the weights don't match; let me keep scanning, and pop up a "weights didn't match at some point" error right before payment.
Now they only need to check, at most, once per customer. It's nice for customers, but pivotally for the stores, it's nice for the employees.
Where I am, those errors pop so often that the employee's job is basically just hitting OK on all those false positivies. They don't check anything in my bags, and I frankly doubt they have the time to do so. Scanning 10 items usually results in at least 2 weight mismatch errors. With something like 8 self-checkout kiosks, there is a large and constant flow of false positives on weight mismatches.
Just pop it up at the end, and I would bet that they'd save enough time to have cashiers actually checking what people scanned instead of being a human bug-fix for corporate's stupid policies.