But it is stupid to allow any old IT staff to do so, and this thread is a good illustration of why: because most IT staff do not have the discipline or smarts to keep what they learn sufficiently confidential. Allowing IT staff to browse the files of other staff at will can lead to other HR problems such as harassment or even blackmail, or loss of corporate reputation if people post embarrassing stuff in, say, a public HN thread.
The ability should be exercised only under the supervision of a lawyer, which limits bad behavior and creates attorney-client privilege for discussions of what might be found.
I worked at a place that did have good 'secrecy' around most monitoring. While I was one of three people outside of Infosec that managed to find out that someone was let go because they were caught exfiltrating client/employee PIFI... I'm pretty sure nobody who was possibly compromised got informed.
This was a place that was so concerned with image that the handbook was about as strict as what my Sister had to deal with when teaching at a Catholic school. Image was everything to them.
I have been using my work laptop quite heavily for personal use and I would prefer not to stop honestly.
I believe my intentions are pure and to provide value, I understand world is not perfect, but I would not want to work for an employer that needed to monitor me.
Lastly, the companies or specific malicious admins might simply not care about the legality and still monitor you - either for company reasons or simply to stir through your data. If they have admin access to your computer, it's simply not your computer.
Every large company I've worked at or heard of it's pretty much assumed that IT may monitor everything you do on their machine. Everyone knew this. Which is why you don't use the company laptop for personal use.