I don't see anything humorous in the example you give. My guess about the meaning of it is that employers are required to compensate their employees for wages they lose during jury duty (if it exceeds a certain number of days) -- generally not as much as they would ordinarily earn, but something. Because there is no one to compensate the self-employed person for lost income, they are not required to serve on juries.
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Stenka25distinction between 'exempted' and 'exempt'Exempt is the adjective.