As pretty much everybody else in the thread has already pointed out, America is supposed to different from other nations in that it allows and encourages (supposedly) the freedom to choose and practice whatever religion one chooses without fear of oppression, so I guess from a purely Constitutional standpoint, yeah, they can do that. Honestly, though, I think it's an unwise move for the Muslims in charge of the development to do so because it shows a definitive lack of tact, which could potentially go a long way in smoothing over the misunderstandings that have separated Islam and Christianity for so long. If they had chosen to say "we understand why this could sting for a lot of you" and picked another spot to build their mosque, it would have demonstrated that the Muslim community as a whole is sympathetic to America's loss during 911 and that they're willing to put aside their own agenda in order to show reverence to the gravity of the attacks on a lot of Americans' minds. As it is, it's just turning into another battle of entitlement with more than a little bit of bigotry and bitterness thrown in from both parties.