As I understand it, Apple increased the peak power usage of the 6S (as a result of making the processor faster), but chose to use the same battery (in order to avoid making the phone thicker). As a result, unexpected shutdowns became much more common - while this did happen on older models, it seems to have been much less frequent.
Strictly compared to the alternative of letting people's phone shut off, throttling seems like the right decision, but I would also argue that Apple produced a defective product, where the battery wore out much quicker than consumers expected (because even if it had sufficient capacity, it couldn't provide the peak power necessary). As a result, Apple's decision seems more like a way to cover up their design flaws, rather than to actually be helpful to consumers.
Strictly compared to the alternative of letting people's phone shut off, throttling seems like the right decision, but I would also argue that Apple produced a defective product, where the battery wore out much quicker than consumers expected (because even if it had sufficient capacity, it couldn't provide the peak power necessary). As a result, Apple's decision seems more like a way to cover up their design flaws, rather than to actually be helpful to consumers.