Comment by rollcat
One good reason why "honest" app vendors do this is because providing tech support for custom OS's (in addition to the wide variety of popular handsets) is more costly. They also might not want the responsibility - in case something like your banking app gets pwned by random malware, they want to blame the OS vendor. CYA is always a good strategy.
But if someone is seriously thinking client-side security works, yeah the app deserves your review - and probably some reversing, just for fun.
It's not hard to think of reasons that are rational and not otherwise nefarious that an app developer would want to restrict an app to certain verified operating environments, but I think creating a world in which people have less control over devices they own is bad in and of itself. I don't run a government or a VC firm so I don't have a lot of power to stop it, but I'll make what small contribution I can.