installing the cert does not imply they understand why they need to do that, or that they must do so themselves. It's not an equivalent for "they have accepted to let him see everything they do".
probably because it actually is, if the users aren't properly informed of its existence. It depends from the local laws of course, but it's definitely something to be aware of.
His opinions won't save him from getting sued to hell and back or from the purely social repercussions (i.e. very angry relatives/guests) of literally snooping encrypted traffic without explicit consent.
In most enterprise environments (every one I've ever worked in), you're reminded on every login to a PC that is under central administrative control.
Try to connect a browser without the enterprise root CA installed and you're not reaching the Internet. Heck, if the MAC address of your device isn't in IT's database of approved devices, you're not even getting an active switch port.