For decades now, in most consumer electronics, ultra lower power microcontrollers always remain "on" (not saying this is a good thing, only that it is factually true). As these microcontrollers become more advanced, they can detect touches with incredibly small power draws.Don't you need "real" buttons to turn on or force restart the device? Otherwise some part of the software needs to always work/run.
There is really, in most modern consumer electronics, no such thing as truly "off".