>(their downfall was neither of them were meeting productivity expectations, not getting their work done)
It's only unethical if you're not in the ruling class. If you're an executive, it's perfectly fine to sit on the board of multiple companies simultaneously (each one paying you $50,000+ per year).
Unless you are somehow working them in totally offset hours, or you have _literally_ zero meetings and work in a completely asynchronous environment, then you need to disclose.
But that's not true for an overwhelmingly large number of people. You aren't just being paid for output, you are also being paid for time.
Ethically though, I don't feel this way at all. They're paying for my output not my time. I don't get paid overtime when I work extra nor do I get credit for working on the weekend.
I don't think I'll convince you but I guess I would say you don't owe your employer your life. They'll cut you off when it's to their benefit and it's best you do what's best for you.
If that employee does all of their work for you, why does it matter if they work another job? Are they underperforming? I'm sorry, but I don't see how that is a scam. In retail, people work multiple jobs all the time. Why is it suddenly unethical the moment it turns into an office job?