Why should they? Of all humanitarian aid goods, only about 20% has been delivered in the past. The rest has been subject to ridiculous bans whose only purpose is to collectively punish the Gazans for voting for Hamas. This doesn't inspire any confidence in the Israelis changing their mind now and dutifully transporting all the cargo, without holding anything back. Ok, there have been promises by Netanyahu, but promises are cheap. It's too easy to delay the delivery and to come up with nitpicking "problems" with parts of the cargo, resulting in the actual delivery being only a subset of the goods once again. A neutral observer may help, but it will take strong pressure on the government for them to accept any supervision. Having said this, imho it's counterproductive for Hamas to weigh in on the controversy. They're not trustworthy, either, and their loudmouthing will only result in the government more stubbornly resisting compliance. And, after all, those aren't Hamas' goods at all, but those of the humanitarian organisations! That stuff is for the people of Gaza, not for their radical "government".
from the article: Hamas blocking entry of flotilla aid into Gaza