I don't agree, if it's the workplace, you are meant to be doing work related issues. Like I said, what is ethical depends on the case. At work, it doesn't 'go both ways' you are given a job, and you are meant to do it, the only time I guess it is NOT OK is if they monitor you and the company doesn't have a policy on using the computers for personal use.
But pretty much all companies do. If you are meant to be doing something, but doing something else, how is it someone else's fault?, if it's their systems, they should have the right to install whatever the heck they want, you being monitored unknowingly is your wrongdoing, because you were doing something that you weren't meant to be doing.