Actually, I don't think it is that simple. Customers have the reasonable expectation that they should be able to use a product for a certain amount of time. In fact some countries have pretty strong customer protection laws, that require that products work for a reasonable time.

Of course, Sparrow will continue to work with Lion and iOS 5. But how reasonable is it to expect that people continue to run iOS 5 if Sparrow breaks on iOS6, missing on all security updates, to get the expected lifetime of their purchase? Or what if it turns out that there is a grave security vulnerability in Sparrow tomorrow?

Suppose that you spend five Euro on a knife. Sure, you should not complain if it breaks after ten years. But if it breaks after six months during normal use, you take it back to the store.