The stores get away with it because even when ignoring the fact that tax is added after, few of the shoppers in these stores will remember the shelf price for a basket of 20+ items from the store. They might remember one or two, but they won't remember (and therefore will not notice) enough of the shelf prices to notice the systematic overcharge at the register. In reality, a good number of the shoppers likely don't remember any of the prices from the shelf tags, and will not be mentally summing up what the final price should be, so those shoppers won't notice the discrepancy at all.

> stores get away with it because even when ignoring the fact that tax is added after, few of the shoppers in these stores will remember the shelf price for a basket of 20+ items from the store

We don't bother remembering it because we're in a high-enough trust society where that burden shouldn't be necessary.