Just to be clear: this is a free country, and Mr. Falwell has a right to say what he thinks, even if his views include the belief that other people, by saying what they think, brought down God's wrath on America. By the same token, any political party has a right to include Mr. Falwell and his supporters, just as any politician has a right to make a political alliance with Mr. Falwell.
But if you choose to make common cause with religious extremists, you are accepting some responsibility for their extremism. By welcoming Mr. Falwell and people like him as members of their party, Republicans are saying that it's O.K. - not necessarily correct, but O.K. - to declare that 9/11 was America's punishment for its tolerance of abortion and homosexuality, that Islam is a terrorist religion, and that Jews can't go to heaven. And voters should judge the Republican Party accordingly.