Almost always, as I've noticed only after grabbing a friend's power bank and a C-to-lightning cable for a day trip...

I believe an USB-C device port is easier to implement than a host/power supply port, since the host is only allowed to supply Vbus after ensuring that it's connected to a device (by actively probing the resistor connected to the CC pins etc.), whereas a device port only needs to present that resistor. After all, even a passive C-to-micro-B cable can do that.