Actually, this isn't entirely true. They COULD have implemented this in iOS 9 for older devices, and just had it be a "dumbed down" version with the 6S being superior. They've done this before too.
iPhone 4 could get turn turn-by-turn in iOS 6, but didn't have voice guidance.
iPhone 4 got iOS 7's design despite not being able to display any of the "special effects".
iPhone 5 could do "burst mode" pictures in iOS 7, but it was much much slower than the 5S.
iPhone 5 could track health data in iOS 8 despite not having an M coprocessor.
I think you get the point lol. Apple has included slightly watered-down versions for the older generations while giving the newest device the best experience before. They definitely could have easily added a front facing flash for the older models, but sadly they wanted it to be 6s-only, probably only to make it have more exclusive features.