A Squared - it's actually a USDA rule, not Hawaii state and it's simply that Hawaii might have its own species of critters that could cause agricultural problems on the US mainland. Whether this is sensible or not is another matter, but there have been some serious issues in Hawaii (e.g., ohia rot and banana bunchy) and guess they're just being cautious. I think the Hawaii State of Agriculture dept does the screening as that's the most efficient way to it. To have the screening done at the arrival airport would, of course, be rather expensive and complicated to arrange as Hawaii flights arrive at domestic terminals.
https://www.aphis.usda.gov/aphis/res...aiian_products