“Doing what they are hired to do” is very often defined in employment agreements as working x number of hours.
Not necessarily true anymore in white collar professions, especially nowadays with gig work. It really depends on the language and terms of your employment contract. I've worked for places that define the employment as 40 hours per week, and also for places that define it as specific tasks for a length of time, and also for places that define it as availability during set hours of the day. It's very important to read the employment contract terms and the company's employee handbook.
You can’t really say you’re doing what you’re hired to do if you take a second job that you perform during the same hours when you’re not allowed to under your agreement.
If your job explicitly defines your employment as being available and dedicated during set hours, or if your contract explicitly says you can't take on additional employment, then you're right. That would be "double-dipping".
I also hated working for those types of places, because they're usually run by micromanagers who failed up and measure their worth by how many emails they forward along. Which are probably the same type of people who are mad about overemployment to begin with.
The way I see it, it only becomes a problem if you have multiple jobs that have a problem with it. And I can't imagine why anyone with the means to work two 6-figure jobs would choose to work for two of those companies.
But that right there is the issue. Why should a company be allowed to prohibit employees from having a second job if it doesn't conflict with the first one? And if a company does have that right, does it apply to all jobs? What is the difference in that case between working two jobs in the same industry in different market sectors vs working two retail jobs?
Another POV: if I incorporated myself tomorrow and offered what I do for a living as a professional service, then I become the company and the companies that hire me for my services become the client. Do clients have the right to say I can't take on other clients? (FWIW I have seen some clients try that and get shut down immediately, and I've also never heard of any company agreeing to those terms with a client.)