There is a definite benefit to giving good faith customers confidence in their ability to return something if they aren't satisfied with it. If the majority of customers are not acting in good faith (meaning never intend to keep the item) that position becomes untenable for the business and as you state the terms are changed. The alternative is to drastically increase the price so that customers who actually want to purchase it can subsidize those that don't.

I'm not sure whether there's an ethical problem with acting in that way, but it resembles a "tragedy of the commons" situation. Or, "this is why we can't have nice things".