If Hamas takes the aid in, Israel will use its propaganda machine to say that the aid was delivered through Israel and the whole conflict was an unnecessary stunt by "terrorists". Pay attention to the Orwellian use of vocabulary, similar to what you see in V for Vendetta. By putting fair conditions to the delivery of the aid (release of captured peace activists and delivery of the complete aid) Hamas is bringing to the attention of the Western public through media coverage that Israel is still holding back a good chunk of the aid, although it is well known at this point that there is no weapons among the aid items. Israel is holding back humanitarian supplies from an imprisoned population. This is really not hard to figure out unless you are completely brainwashed by Israeli or US media.
Arab MKs postpone Al-Aqsa visit planned for Sunday (Haaretz)
from the article: Hamas blocking entry of flotilla aid into Gaza