Because it is the technical definition of the words and that is exactly what it is. Something not being illegal does not mean that it isn't fraud or stealing. Misrepresenting your availability and willfully trying to cover that is is fraud. Taking someone's money while fraudulently not working for it is stealing. I know overemployment people will rationalize it some other way but it doesn't change that.
If those so "overemployed" (that is to say, working multiple jobs, a normal and common thing to do in society) were not working for their employer, they would be dismissed quickly.
I think you are conflating "working multiple jobs" with working multiple jobs during the same time period and lying to the other jobs about what you are doing when you cannot be reached. There is a big difference. If I work 9-5 on one job, and 5-1am on the other job that is not taking time from one to spend on the other. If I work 2+ jobs during that 9-5 period and not tell anyone and make excuses for not getting things done long enough until one of them fires me that is being dishonest.
