"Instapaper Free always had worse reviews in iTunes than the paid app. Part of this is that the paid app was better, of course, but a lot of the Free reviews were completely unreasonable.
Only people who buy the paid app — and therefore have no problem paying $5 for an app — can post reviews for it. That filters out a lot of the sorts of customers who will leave unreasonable, incomprehensible, or inflammatory reviews. (It also filters out many people likely to need a lot of support.)
I don't need every customer. I'm primarily in the business of selling a product for money. How much effort do I really want to devote to satisfying people who are unable or extremely unlikely to pay for anything?"