There is a plausible economic incentive to do this:

Reputation.

This happens less in markets with few, big sellers and lots of customers locked into long-term contracts (like ISPs), but it does happen occasionally in high competition markets where customers can take their business elsewhere easily.

Restaurants are a good example - where I live, a host might hand out a round of after-meal shots on the house to encourage a big table of uncomplicated guests to come again.