Constitutional Clause
First Amendment reads "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof". The atheists (not liberals) were protesting the use of tax money to pay for the cross to be included in the memorial. If a mosque could not be built two blocks away on private land, why should christians add a cross to public land? This could be interpreted as the government being pro-christian and anti-muslim.
The constitution provides for freedom of religion and it would help that we can have freedom from religion, as in not having someone's beliefs forced onto them.
In either case, if a cross is allowed, why not a religious symbol from every religion? And, if used in its original Hindu context, why not a swastika?
I am not a muslim, I am an agnostic.