If you are salaried, then you are by definition not getting paid by the hour. Most employers and employees don't understand that. My employer is an idiot at times, but I will never forget this one meeting...
A couple of my coworkers complained that they were working longer hours and that I was leaving early at times. My boss printed out a report that showed everyone's production numbers for the year and handed it them out to everyone in the company. He said all that matters is production, if it takes you 30hrs a week or 60 hrs a week it's irrelevant. I had the highest production numbers in the company. He asked those who had complained about amount of hours worked if they had any questions or comments...crickets. if your job is based on production and you are producing what your employer expects you to, then what else you do with your time is irrelevant. Good employers realize this, f*ck bad employers.
With some jobs, this is easily demonstrable and with others it isn't.
So yes, for jobs with discrete deliverables where the employee has no other responsibilities, this is exactly the kind of job where taking on a second job would be doable because it's self evident that the person is meeting the demands of each job.
Not all jobs are so neatly organized though. I've frequently terminated my highest producers because they were overall terrible employees, but that's because they were roles where production rates depended on a lot more than individual effort and skill.
It's really convenient to assess staff when there's a clear, measurable, 1:1 ratio of skill and effort to outcome. It's often not so nice and tidy though.
Also, although being salaried means not being paid hourly, a salaried worker can still be contractually obligated to be working during certain hours.
I agree with you that there's generally WAY too much emphasis put on controlling employee time, but it's not as cut and dry for all job roles as just looking at measurable product output for an individual.
Sometimes it is, but often it isn't.