in the post you replied to
I referred to a lack of courtesy by IT departments. The example I gave IT interruped a technician doing legitimate company work without consideration of his time or how checking to see how urgent he needed to complete what he was doing. The time you're at work belonging to the company should have nothing to do with respect and courtesy. I know you regard workers as slaves while they're on the clock, but it shouldn't be so.
Abover that I pointed out that legally anything you do on company time and/or equipment was the business of the company. Partly it's there business just because it's their equipment and time. The company can also be held legally responsible for what you do use their equipment, and sometimes what you do while on their time.
My last job we had time off the clock for lunch. You didn't have to bother to punch out if you ate on site, they automatically adjusted for the time off. However, you were required to punch out if you even drove across the road to get a hamburger. Why? because if you had a wreak and time records showed you were on the clock it might end up involving the company in the responsibility for the wreak.
When you're at work you are suppose to be "on the job". You have a responsibiity even during your breaks. If you have lunch off the clock you should have a little more privacy and leeway, but that doesn't necessarily mean free use of the company equipment. That depends on company guidelines.
Your example the boss certainly has the right to check your work. Interrupting your work may be against the company interest depending on why he interrupted it and may be wrong.
The IT guy commenting on someone's purchase may have been just making a point, if rather snidely, about the employee misusing company equipment. Technically he should have reported it and the employees immediate supervisor deal with it if the employee was violating company policy. His actions can make you wonder if he is just being a busy body and spending more time snooping than doing his job.
A company owns your time at work, but inspite of some beliefs to the contrary, not your soul.