Reproducible:
Always, on my laptop, due to design (code)

Steps to reproduce:
1. open gnome-control-center, page "Power"
2. scroll down to "When the power button is pressed" and try to configure it (choose an option)

What happens:
No option to shutdown or asking for shutdown is available.

What should happen:
A power off button should really default to power off (shutdown) the system. This is the default UX expectation of an user for a power button. In case you really don't want that there must be an option to configure it to shut down the system or at least be asked.

Affected version:
gnome-control-center 3.20.0
gtk 3.20.0

Additional meta info:
It looks like this overlaps with another bug report which is closed as "RESOLVED FIXED", which seems wrong to me: https://bugzilla.gnome.org/show_bug.cgi?id=757880

Additional info aside the obvious UX expectation:
tl;dr: hibernate and suspend is often error-prone causing data loss, it should not be default or even hard-coded.
Hibernate (suspend to disk) would be nice to have by default in an ideal world.
In a real world (as of 2016) many devices have issues going to standby or hibernation. This affects all major operating systems, including (but not limited to) Linux.
If a system fails to suspend or hibernate will lead to these states depending on hardware and firmware:
* just power off without a clean OS shutdown ⇒ risk of data loss
* unable to suspend or hibernate, getting back to lock screen immediately ⇒ UX break
* firmware or kernel is unable to wake up, getting stuck ⇒ user needs to hard reset system ⇒ risk of data loss
Another reason being unable to hibernate is full disk encryption.
Even if a system is able to successfully hibernate or suspend, it will still consume power. In case of a laptop, suspending it (instead of shutting it down) will empty even a full battery after less than 8 hours (e.g. over night) in most cases. ⇒ wasting energy, risk of data loss.