Because it is very likely that the app in question can simply be pulled and installed on any Android device. I have a collection of Samsung phone apps that are supposedly only available of Galaxy S2 and S3 phones. The only ones I keep installed are some of the Live wallpapers. But I have a lot of samsung-specific apps, including launchers.
When buying a phone I would worry much more about the hardware than the software. You can always upgrade software later. But you will never be able to upgrade hardware.