coz the normal behavior of a button is that u have to press it once, not hold. when it's "hold", i feel like it's a special function e.g to force shutdown. Just like our ps4, holding the power button will prompt u "rebuilding database stuff because u did not shutdown correctly".
If you hold the ps4 power button when it is in the "rest mode", it is actually the correct way of turning off the console without having to switch on the tv and going thru hassle of navigating the menu to off the console.