An employee having another job is basically none of the employers' business.
Of course, if an employee's being useless, perhaps warn / eventually fire them, irrespective to whether they have a second job, which again should be none of the employers' business.
It is really difficult to objectively evaluate performance. If nothing else, evaluators have their own inherent biases. Multiple paychecks is an objective measure. It helps determine whether a borderline employee just needs help and support or had chosen to underperform.
I believe companies have a duty to try to help struggling employees. The corollary is employee’s must do their best for their employer.
If an employee is doing a great job, they can have 50 simultaneous jobs. I don’t care.
Of course, if an employee's being useless, perhaps warn / eventually fire them, irrespective to whether they have a second job, which again should be none of the employers' business.