How does this policy regarding disability insurance make sense?

I have some medical issues going on that are lifelong and I want to look into getting disability retirement benefits through my employer (State of Wisconsin). I read over the disability retirement benefit packet and there is an issue I see on page 10… Before I get into that it appears if you get approved for disability retirement benefits you are not allowed to make over a certain threshold of income: https://etf.wi.gov/benefits/benefits-provided-etf/disability-benefits

So into my question: I am considered protected since I am either a law enforcement or firefighter. I see the page 10 of the packet and see I can go beyond the income limits of other people if approved for the disability retirement benefit. "…because you could no longer perform the duties of your ptoective occupation position". So if a person can perform another job's functions than those of a protected employee (say a job like Walmart greeter) they would not be disabled and therefore would not be eligible for the benefit at all… See what I am getting at here? I cannot even think of an example of an ailment that would be so disabling you cannot perform the job of a law enforcement or firefighter but then can also perform other work. (Page 10: https://etf.wi.gov/publications/et5107pdf/download?inline= ).