It’s not justice towards the employer, but justice towards your peers, both those who you work with directly, as well as those who are negatively impacted because you took a job that could have been someone else’s.

If you want to have multiple jobs at the same time, there is a vehicle available for that, it’s called “consulting”.

I don’t think anyone should have loyalty towards their employer - you should be free to jump ship to a better gig whenever you want, in the same way they are free (in the US) to let you go at their convenience. But taking multiple full time jobs is wrong, imo.

What if they have only one job but still perform on the level you’ve described?