1. If you're upfront with it, and everyone involved has signed off on it, it isn't ethically wrong. Its not the overemployment that's the problem; its the deceit. I've seen this happen multiple times, including once myself. Communicate, set boundaries, be a professional. It isn't common to be fired for asking if working a second job is within the bounds of your first job's employment. On the other hand, if you're already working the second job, and you inform them about it; that's deceit.
2. I'm not aware of anyone who is the CEO of 4 companies; well, except Mr Musk, but don't you dare say for a second that no one is batting an eye at that. Most CEOs I know barely have enough time for one company; and obviously the performance of Musk's companies recently suggests he's in the same boat.
3. The original poster pretty clearly inferred that, in these situations, generally speaking these workers are not meeting performance expectations.