Here goes the reasoning:

1- 19 terrorists who were Islamic killed ~3000 people of many faiths and nationalities (generally accepted, with the caveats that they were not uber-devout Muslims and represented an extreme political anti-American viewpoint and an extreme Wahabbist sect)

2- Therefore, they represented all of Islam (News to me ! Broad brush much ?)

3- Islam killed Americans and attacked the USA (let's just ignore the dead Muslims and, no, 19 men who were Islamic did, not 2 billion Muslims)

4- Park51 (aka "Ground Zero Mosque") is a mosque (not true, a community center with a prayer room)

5- X amount of square feet/meters/miles surrounding the old WTC site is "hallowed ground" (despite the fact that NO ONE made a serious effort to buy up this land and the fact that several decidedly not-so-respectful businesses are in that mythical area)

6- Since Islam killed Americans and Park51 is a mosque, the nasty Muslims two blocks away from the old WTC site are a menace/insult to Americans and the families of the 9/11 dead (note my extreme )

7- The feelings/unresolved grief of SOME of the 9/11 dead's families trumps the First Amendment rights of this religious minority (not legal but when did that matter to the opposition !)

8- Therefore: despite a) the incontrovertible fact that the property is theirs; b) they have the right to build a community center or mosque or both; c) they have complied completely with all the local zoning laws; and d) their Imam has cooperated with both the FBI and the Bush Administration.... the Muslim minority loses to the feelings of some of the 9/11 families and of the RW political manipulation