Most employers have verbiage against moonlighting, though not for any benevolent society serving reason of course. In practice the system is majority unaccepting of working multiple jobs.

If there were more jobs available than there were active job seekers, you’d be correct that no one gets hurt. In fact, it would be a net benefit! There are also highly skilled labor categories with thin applicant pools where an individual working a second job may be the only qualified candidate. There are certainly exceptions.

For the record, no one blames people for being people and looking out for their best interests. Just don’t ask me to defend the policies that allow it. The same policies that stagnate the economy and drive wealth inequality.