[jen20]

It's not done for either of those reasons. It's done to remove the decision point around believing you need to comparison shop on impulse purchases, by pretending that a price will be matched later should you find one. However, the terms are usually such that they will never be honoured.

[BigGreenJorts]

Many groceries near me do price matching, but indeed they limit it to a select few competitors (the big 5's brands in the same price market)