Actually, those aren't blocked. Well, unless you consider missiles to be humanitarian aid.
Should they just let Iran ship weapons into Gaza to be used against Israeli civilians?
I am sure that if no missiles were being brought into Gaza or if those missiles weren't used for killing Israeli civilians, there would be no blockade of anything.
What do you expect the Israelis to do? Just die? Oh wait, that is what you expect them to do.
The flotilla organizers refused this offer. Their purpose was not to deliver humanitarian aid, but to cause a spectacle drawing attention to the security restrictions imposed on the Strip.
Once the flotilla was diverted and the contents were unloaded and placed on Israeli trucks to be transferred to the Gaza Strip, Hamas refused to accept it. This humanitarian aid was apparently so urgently needed that Hamas decided they didn't want to lose face by accepting it.