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.

Sadly almost every mobile bank app in my country does this.

What percentage of rooted/non-stock OS users do you think are people like you, vs bots? I'd love to see the numbers if anyone has them but I suspect it's pretty lopsided these days.