apple seems really bad at coming up with dpi agnostic solutions so this is what you get. of course it's a selling point on some level, even if it means more pixels to push. of course at some version they're going to have to address this, but not this year(it's rather ridiculous that you can't configure osx to be at all usable on a screen like on the ipad though!).
I think the more important factor is maintaining aspect ratio and size, at least within product lines (iPhone/Touch and iPad).
When designing an application, being able to standardize your UI elements to a fixed display size makes things easier and is friendlier to the consumer. For example, if Apple increases the size of the iPhone display at some point, then it's possible that UI elements designed for the new display could be too small for comfort on an older one.
This was a problem for my dad when he started using a home PC with Windows and he had to reduce the display resolution so that applications operating in a fixed resolution smaller than the size of the screen would be easier to see. This applied mainly to specific shareware programs he liked but it complicated his user experience in that he had to learn to adjust properties of the OS about which he should never have had to learn. In an OS with few customization options, those adjustments might not even be available to the end user.