You're right, and I forgot about this until you said it. My recollection is that about 45 minutes before landing in HNL, all of the airline's regular audio and video were cut off and they played a video from the State of Hawaii that welcomed you to the islands and talked briefly about the ag process. Then the stewardesses handed out declarations forms that were similar to the customs and immigration forms you fill in when you fly between the US and Mexico. I do not recall to whom we handed them, but I do recall that our bags were not inspected.

Perhaps the reason for the stricter outgoing inspection is that maybe Hawaii has native parasites that they want to prevent from spreading. It's similar to travel to Newfoundland by car. When you go TO Newfoundland, you just drive off the ferry and onto the Trans-Canada Highway, but when you LEAVE, they will hose off your car, including the underside, and ask you if you have any potatoes; also, if you are driving an RV, they may ask to look in the refrigerator. The reason is precisely what I said earlier--some sort of disease in the potatoes there that would devastate the rest of the provinces' potatoes if it spread.