The article is just clickbait, looking for an angle. Whether or not an employee is allowed to take on outside work is industry and company dependent. Since most do not allow it, there's a prevailing expectation that it's "just not done". Except sometimes it IS done. Here's my situation:
I'm in the U.S.
I'm a W2 employee.
My job is unionized.
I have an employment contract.
Twenty years ago, a supervisor came to me and said, "Your boss is going to [X]. Are you going, too?" I said, no, I already had a job. He said, "Double-dip, baby! That's what [Boss] is doing!" So, I did. Both were W2, and the additional job paid 3x what the first job paid. In fact, I think I was allowed to take the additional job because the first job had a "More Remunerative Employment" section of the contract, meaning I could take an additional job if it was more money, but not less! These were both long-term jobs.
Now I work for a totally different employer. My company and my union both allow and even encourage taking on additional employment whenever possible. I only take the short-term stuff because doing it long-term is brutal. Fortunately, short-term stuff is usually what's offered me, because this is all upfront, so everyone knows I really work for [MegaCorp], whose brand is kinda why these outside employers want me in the first place. MegaCorp views outside work as a way to get free networking and free employee development. Generally, it's 1099 work, but sometimes it's W2. A former boss used to throw extra jobs my way that she couldn't do herself. Shit, sometimes I'm ASSIGNED these jobs, because it's for one of our subsidiaries. I have the right to refuse, but it's double the money, so I don't mind.
You see, my employer pays me to produce [X] by [date], and I'm 2-4x faster than everyone else, so I have a lot of time on my hands. MegaCorp doesn't want me to spend that extra time being unhappy, looking for places to insert myself and sowing discontent, nor do they want to think up busy work for me, nor do they want to give me a raise. Letting me take other work solves all those problems.
Where working 2 jobs gets hinky is if there's 1) deception, and 2) deliberately producing an inferior product because you're over-committed. Those things are NOT COOL. But, if everyone involved is a competent, functional human being, then it can work out very well for all parties.
My takeaway from reading the comments on this thread is that a lot of people are misclassified as salaried employees.