I don't understand why Apple has not added a passcode requirement option to turn off the phone.

Probably because iphones(like any other phones on the market) do crash sometimes, and the only way to bring them back to life is to do a hard reset(usually hold the power button until the device shuts off) - it's a necessary engineering feature.

Holding the power button on my device (not iPhone) restarts the device, not shutdown. I think that would work too.

Perhaps the phone could turn itself on again intermittently?

Not being able to remove the battery could be an advantage in this case :)

There could of course be downsides to that.

1) in situations where phones are not meant to be switched on (e.g. some flights)

2) from a privacy perspective not being able to ensure your phone is actually off is not great.

You can't be sure that your phone is off already. Phones can be remotely activated and used as eavesdropping devices.

Is this really true or your speculation? Really curious about that but yeah, I am also suspecting that while the phones seem quiet they are actually listening ... sometimes.

In 2014 Snowden said NSA can do so (but not how).

It was quite discussed at that time. for example: http://www.tomsguide.com/us/nsa-remotely-turn-on-phones,news...

That's not even close - you linked to an article from 2006 (before the first iPhone was introduced) about microphone activation on phones like Motorola RAZRs which are already powered on. Oh, and this was legal and in the open.

Nice try, but that's a long way from a (long running, secret, and still hidden) vast government/corporate conspiracy to remotely power on any smartphone to spy on the owner.

Anyway, I guess a thief could just wrap the phone in tinfoil :)