I Dont Get It

For a few weeks now, everyone that wants to be heard has been saying that if Hamas wins the election, then all negotiations are off. The White House has issued a statement that they wont deal wth Hamas. Acting Prime Minister Ehud Olmert has said that Israel 'cannot allow' the radical Islamist faction Hamas, in its current form, to become part of the Palestinian Authority.
Why?
Why would we negotiate with Fatah, a terrorist organization that became a political terrorist organization, and not Hamas? That seems a bit hypocritical to me. No one truly believes that the PLO stopped using terror once they became a political organization. Arafat hadn't even offically "renounced" terror when he spoke at the U.N. So why the big deal now that Hamas is stepping in? Why not let them become a political terrorist organization too?
It doesn't really matter because the reality is that Hamas has around a third of the votes, and we will negotiate with them, because we always negotiate with terrorists.