I get the convenience part so the staff doesn't have to go around do it by hand, but it just seems infeasible to do it for the other examples mentioned.
E.g. you go in, pick up item listed for $10, finish shopping in 20 mins, item now costs $15 at till.. probably leave it (so now the staff has to re-shelf it) and start shopping at a place that is not trying to scam you.
For the other example, if there are a few packs of something expiring and they reduce the price for all the items on the shelf, everyone will just take the ones which have a reasonable shelf life left leaving the expiring ones.