Hasn't the safe advice been that whatever you do on company resources (except time) can be monitored/investigated by the company? Isn't it generally bad practic

Hasn't the safe advice been that whatever you do on company resources (except time) can be monitored/investigated by the company? Isn't it generally bad practice to handle personal business on a work phone/work laptop/work desktop anyway due to the assumption that your activities can be legally monitored by your company? Understandably, if you make a send a message/make a personal phone call on company time to your doctor or spouse regarding some issue then that shouldn't be grounds for the company to investigate just because you were on their time and my reasoning for this is that even when you're off the clock, you can still be disciplined for activities that may portray the company in a bad light (getting into a bar fight after work, for example) so its assumed that you're never really off work. Either way, if I were him I wouldn't have assumed I could have used company hardware to send personal messages yet still have an expectation of privacy.