I worked for a UK subsidiary of a US company. Right on the login screen was notification that all use and communication was subject to monitoring.

Why would it be otherwise?

You are paid to do the company's business with the equipment they provide. It's not a matter of 'power'.

Imagine a UPS driver taking the van to run errands for his friends. Few people would argue that it is inappropriate for UPS to track the van and fuel usage. Maybe he scrawls anti-management missives and keeps them in the glovebox. Why is IT different?

Just because something is inappropriate at work, it doesn't follow that your employer should have carte blanche to breach your privacy on the off chance that you are doing it.

Yes, people abuse freedoms all the time, but to many a lack of privacy is a bigger threat.

Imagine someone who is breaking the law at work and someone who is breaking the same law in their home. Just substitute "employer" for "government" as a reason why people should be constantly and fully monitored without any probably cause. It's basically like saying everyone's electronic communication should be monitored at all times to see if they are feeling sad, angry, etc. which might put them at risk of committing a crime.

The right to privacy is fundamental to freedom and it shouldn't stop once you walk in the workplace.