That only shows that it is unethical for you to use the company's computer irresponsibly. It does not show that it is ethical to monitor the computer without your knowledge (e.g., the ethics of duty here could apply in both directions).
If I was at work, no. I actually subscribe to 'responsibility' if I was caught doing something at work and I didn't know they were monitoring, it's still my fault, not their fault.
You're missing the point: I am not arguing that it is the monitoring that is unethical, but the lack of prior warning that is unethical.
If I was at home, yes, because no needs to monitor me there, unless I was doing something wrong again, like MESSING UP the family computers.