All anecdotes I see about this dude is: "we hired him and he did a fantastic job, but once we found out he had multiple employment we fired him".
... why? If the guy's doing well by all metrics and not leaking IP, literally, who cares?
He's not going to get much sympathy. Because:
1) from the employer side, this runs afoul of all MBA theory and practice, so he could have been more profits. Almost by definition, this means you're not getting the maximum out of the guy. Oh and there's jealousy of course.
2) from employee's side, this runs afoul of union thinking. Those jobs could have employed 5 people, maybe more. Oh and there's jealousy of course.
I saw several anecdotes that were: “when he did the job, it was great, but he rarely did the job because there was always someone sick, meeting with a lawyer, or any excuse to not deliver”.
So I think that finding about multiple employment is actually about realizing he was lying the whole time with the excuses.
We had a guy like that... the thing is you cannot really pass any responsibility on him because he will eventually be distracted with other stuff. You will never know when you have him 100%. You don't want to keep checking on your employees week by week day by day, if they deliver.
You need some degree of trust in your employees (you cannot "verify" all the time), and you cannot trust some guy you KNOW is cheating on you.
This shouldn't come as a revelation, but it's risky to employ people of low character. There's the risk of theft, lawsuits, etc – not to mention, nobody needs the frustration of dealing with lies and flakiness.