Blue collar workers are almost always paid hourly (or some other unit where time = money). They're being paid in proportion to the amount they work, and the only common reason to take a second job is that the first won't give you enough hours/work to meet your financial needs.
Some people see salary jobs as exchanging a fixed amount of money for 40 hrs/week average. If you're spending 10-20 of those hours moonlighting for company B, those people would say you're depriving company A of what they're paying for.
If you instead see salary work as producing a certain amount of work regardless of the hours worked, then again there's no issue here. This is inconsistently applied to executives far more than rank and file office workers, since no one really expects (or wants) board member Bob to provide 40 hrs/week to each of the 6 companies he's involved with.
A manager should have at least a vague notion of how long a given set of tasks should take an employee.
If I assign 20 hours of work and it takes 40 hours, I should not be surprised the employee does something else with the 20 remaining hours.
If I assign 40 hours of work and it takes 40 hours, and is of the expected quality, I really don't care if the employee take a part-time job elsewhere (assuming that doesn't conflict with expected online hours, etc).
You are either managing factory workers, or living in a dream world.
My boss has no idea what I'm working on, day by day. My assignments take as long as they take; he makes WAGs at the project start, and I get informed if I am burning more hours than he expected.
But at the end of the project, a "40-hour" project may take me 10, or 100 hours. Or, rarely, 40.
But overall, I think I have a feel for how hard my employees are working and whether they're completing things in a time I consider reasonable. Maybe I'm being played. I dunno.
Terms like 'moonlighting' get a bit loaded. If I'm not competing and happen to turn my model train hobby into a profit-making venture, who is my employer to say what I do in my time off?
I've found most employers want to rely on the vague bounds of salary. Finished when the work is? Well, surprise: the work is poorly defined.
Some people see salary jobs as exchanging a fixed amount of money for 40 hrs/week average. If you're spending 10-20 of those hours moonlighting for company B, those people would say you're depriving company A of what they're paying for.
If you instead see salary work as producing a certain amount of work regardless of the hours worked, then again there's no issue here. This is inconsistently applied to executives far more than rank and file office workers, since no one really expects (or wants) board member Bob to provide 40 hrs/week to each of the 6 companies he's involved with.