That's because both situation have absolutely nothing in common.

People tell cashiers when they make a mistake not out of a desire to avoid "stealing" (stealing is taking something without permission or right by the way so neither case is actually stealing by the way) but because they empathize with an actual human being making mistakes like they sometimes do and want to be helpful.

Corporations can't at the same time replace people with machines to optimise their bottom line and expect their customers to remain empathetic. Once you put in place automatic system you have to owe them. You can't win on both side.

If Amazon gives huge reductions by mistake, well, too bad for Amazon, the reductions are still there. It has absolutely nothing to do with shoplifting.