But, I see a lot of emphasis being placed on the fact that it was a company-owned machine and that it was ostensibly being used for company purposes. These two facts seem to underpin a lot of what everyone says.
But ownership and mode of use are not the only kinds of resources or properties to think of. For example, at least in the US we have the concept of "company time" which covers the time periods during which you are performing an action sanctioned by a company. Could this be used to demand the ability to monitor employees during time periods when they are on company time?
Likewise, there are a lot of gray areas going in the reverse direction -- from employee to employer. For example, it is true that HR records (say, for example, performance reviews written by my boss about solely me) are electronic files that are the property of the company, just like a computer workstation is property of the company.
But, just as a computer workstation could be misused for personal use, and thus needs to be inspectable for compliance, so also the HR files could be misused for personal use (making jokes, talking about non-professional aspects of me, conspiring to deny a promotion simply due to personal preference, etc.) -- so why aren't such files open for inspection?
Yet another interesting aspect of all of this is why we believe that not only must the workstation be made available for inspection, but further that it is the employer who gets to perform the inspection.
Why isn't such inspection required to be handled by a third-party arbiter, much the way that tax and official records are required to be kept by a certified firm specializing in it. We don't trust companies to say, "Yep, these are my taxes and records, I swear I didn't lie or hide any money." We at least require some third-party to attest to that and stake a reputation on it (subverted though it may be).
Yet with issues of inspection, we just sort of capitulate to this idea that the employer not only gets to raise the issue of suspicion about misuse, but also gets to be the authority on the investigation of whether or not misuse occurred, and to what extent employee privacy must be invaded to get all the relevant info.