And that's probably because they don't want to include the Reboot option in the power menu [1].
[1] http://www.androidpolice.com/2015/12/02/sony-is-trying-to-pu...
[1] http://www.androidpolice.com/2015/12/02/sony-is-trying-to-pu...
On the devices I've worked with, that button handling is done by the power management silicon and not by software at all.