You can justify this on military grounds, of course, though I won't; I think collective punishment is never warranted, and sieges rarely, and certainly not in retaliation for rocket firings that killed, IIRC, one Arab, and the kidnapping of a single Israeli soldier. And I certainly agree that Israel's critics do not lavish enough attention on Egypt's enforcement of the same blockade, though Egypt does let the leash slip every now and again.

But whether or not you agree with the policy, this was not particularly about keeping Hamas or other groups from getting weapons--the "weapons cache" found aboard consisted of knives, slingshots, and wooden batons, which pose no threat to Israeli civilians even if they make it to Gaza. This was about control. And it backfired badly as the crack Israeli troops either opened fire on a poorly armed group of civilians, or, alternatively, moronically dropped one-by-one into a violent mob of poorly armed civilians and got their sidearms taken from them. Either way, they did not get the overwhelming display of control that they were clearly seeking; instead they got this graphic, which has been making the rounds in military and defense circles:

You cannot understand the Israeli-Palestinian conflict if you are determined to believe that every single thing one of the two sides does is the brilliant and imminently necessary exigency of an existential conflict, the brilliance undone only by the perfidy of a biased media that refuses to tell the true story. People, especially large groups of them, are more complicated than that. And both sides in this conflict are attempting to play a long game. To my mind, at this point both of them are playing extraordinarily badly. But that's a blog post for a different day.