Because Revelation places a strong emphasis on Israel during the tribulation, and not on the church, most post-tribulationists have adopted a replacement theology view in order to maintain the focus on them.
Replacementism is the view that Israel, having failed God, has been replaced by the Church. The Church is now seen as spiritual Israel and spiritual Jerusalem. This teaching claims that all the promises and blessings, in fact Israel's entire inheritance, now belongs to the Church. However, all is not lost for Israel; it gets to keep all the curses.
Dispensational theology, taught by nearly all pre-tribulationists, teaches that God has separate strategies for dealing with the Church and the Jews. When you consider the change in focus, during the tribulation, from the Church to Israel, the pre-trib rapture provides a good explanation for this transfer of attention.
To say that Israel is no longer God's chosen people is really playing with fire because the Antichrist will likely be saying the same thing when he tries to destroy the Jews during the tribulation. I look for people that hold to replacementism to be in the cheering section when the Beast goes on his Jew-killing campaign. "The Lord will not reject his people; he will never forsake his inheritance" (Psalm 94:14).
"This is what the Lord says, he who appoints the sun to shine by day, who decrees the moon and stars to shine by night, who stirs up the sea so that its waves roar - the Lord Almighty is his name: 'Only if these decrees vanish from my sight,' declares the Lord, 'will the descendants of Israel ever cease to be a nation before me'" (Jeremiah 31:35-36).
Now I see why you get so frustrated with these guys Adam.