But more to the point: the logistics of making refunds on purchases like software are complicated. Most payment processors don't even have the notion of partial refunds.
Making a full refund is costly to you (payment processor took their cut of the transaction; they aren't too eager to give it back to you so either they charge you (and now you have a loss) or they swallow the charge but will kick you out of the system if they have to do it too frequently).
There might be more bad will generated by contacting the customers and offering them refund because that makes them aware of some event that they might not like.
Most people don't monitor the price of software they bought over time so they are not aware of changes so they are not upset about changes. I've bought a bunch of software over time and I can't tell how much any of it cost - I just don't keep track.