They don't actually have to pay him after three days, in most cases.
You may want to look at http://www.mass.gov/courts/sjc/jury-system-b.html
The system isn't great for many jurors, and your husband may not have the rights that you think.
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The employer's expectation certainly seems inconsistent with good citizenship. We are all expected to give triennial jury service our full attention, as befits the solemn nature of our responsibility to help administer justice in our community. Firing your husband for prioritizing his jury service might fall within the narrow category of causes for termination that would be considered illegal, but understandably you don't want to be the test case for this scenario. Perhaps the ACLU can advise you or intercede with the employer to rethink its position. Their Boston office can be reached at 617-482-3170.
Disclaimer: This site exists to provide information only. It is not legal advice. Answering your question does not create an attorney-client relationship. I am a Massachusetts lawyer. Any information provided on this site does not, except as explicitly stated, imply familiarity with laws or procedures peculiar to your state which may differ from those where I practice.