Oh agreed. I just think that there are some pretty big assumptions being made about what an individual with a side hustle is capable of achieving.
Speaking from personal experience, my promise to my employers was that they were my priority and that I would not allow my side hustle to affect my job performance. It never did.
There is also a lot of room for subjective opinions about what it means to "do the best job you reasonably can." Does that mean showing up on time, supporting your team, being available during work hours and producing high-quality work or does it mean giving up on work life balance, being on call and working overtime routinely?
It might be my age, but I wouldn't accept a position that expected the latter.
But, again speaking from experience, when I continued to operate my business while working full time it came from a place of ambition and wanting to do the best for myself and my family that I could. I worked hard, kicked ass at work and made a lot of money that put my family in a really good position.
Obviously there are lazy leeches and employers need to be on the lookout. That's why I prefer that job performance be the metric. Not whether or not someone has other sources of income.
In fact, to put on my dark cynical hat for a moment, I think that one reason employers might not like employees with other sources of income is that it means the employee isn't as dependent on the employer and so they don't have the same power dynamic. There are a lot of nasty implications there. I prefer leading through inspiring and having a work environment that is pleasant and fun.