Actually, Christians using the cross as a symbol of their religion was an impressive piece of propaganda, especially at the time it was done. Remember that crucifixion was used as a method of execution by the Romans, so Christians claiming it as a religious symbol effectively prevented the Romans (or anyone) from continuing to use it. After all, when's the last time anyone heard of someone being crucified? The cross hasn't been used as a device of execution/torture/what-have-you for a long, long time.
That being said, I do agree that there's no reason to put a cross up at the 9/11 site unless you put up a symbol to represent those of other religions, including Islam, who had people die there.