Why Israel cannot do 'prisoner exchanges'

On the one hand, 'moderate Palestinian President' Mahmoud Abbas Abu Mazen pressures Israel to exchange prisoners terrorists for kidnapped IDF Corporal Gilad Shalit so that the Hamas 'ministers' and 'legislators' will be released from jail on time to join a 'national unity' government. On the other hand, Shalit's father pressures the government to do the same. In the meantime, Israel gets a