some work is not about volume but about quality. and exactly the fact that the work can not be measured is the problem. some work can not be done effectively when you are tired, and some work is even dangerous. driving for example. paying attention to safety and security requires you to be a wake and able to focus.
as a programmer i have good days and bad days, and noone can say whether my work is the best i can manage or is slacking because i have a second job. and if i am spending more than my market rate salary as a programmer affords then i need to change my lifestyle, not get a second job.
in germany for example the laws for the maximum time of work an employee is allowed to do in a day covers all jobs. two full time jobs would simply not be possible because you are required to have 11 hours of break between two workdays.
nobody can work 16 hours a day and be focused all the time. heck, most people can't even do 8 hours of focused work. i am not paying you to sit on a chair, but to actually pay attention. if you do two full time jobs, you are not doing the work at the required focus.
sure, there may be jobs where that doesn't matter, and there are jobs that are paid full time but aren't actually full time jobs. like a sysadmin in a small company, where our boss told him that his job was to keep the servers running. if he has nothing to do he is welcome to goof off. and in such a job he could also have used the free time to do side jobs, or a second job.
but for most jobs it does matter that you are focused working for the whole time, and if i find out that an employee is doing a second full time job then i will be very much concerned at that persons effectiveness.