Sure, customers could be rude(I wish they were not but some people have a bad temperament or have something else going in their life and this was a tipping point) but did they wake you up in the morning and forced to create that app and give it to them for the price you sell it? I am sure that it's the other way around, you made an app that promises to solves a specific problem and you offered someone to solve their problem for that low price and they are unhappy that you did not kept your part of the deal.
I would be very pissed off too if the developer of a product that I bought decides that it's not a good business and makes the product unusable. It's not my job to calculate the fair price for the product, the product developers must do the calculation since they come up with the offer in first place. If the deal is made, I expect to to receive the full promised benefits of the product I bought.
That's actually why we have customer protection laws.